I agree that the expressions are not all that similar in terms of their form and the literal meaning of the words they contain. However, the dictionaries I’ve consulted use them to explain 今一つ, and one of them listed one of the meanings of もう一つ as being appropriate 「もう少しというところで」, which is literally ‘in a situation/at a point where one says “a bit more”’, and less literally ‘a “just a bit more” situation/point’. That was the moment where I realised that at least some of them (at least based on what I’ve seen in anime) are expressions that one can use verbally as a form of encouragement or in order to indicate that additional effort needs to be made. In that sense, they are all similar, because they share a use case i.e. they’re all used in a similar way and in a similar context.
You can see it that way, but strictly speaking, あと (often written 後) simply means ‘after’ or ‘later’ in this case, which is then translated into the idea that ‘after [this point], [just] a bit/little [is necessary/desirable etc]’. もう、あと and いま are all time words, and I strongly believe that all the phrases we’re discussing are simply extensions of their temporal meanings. By the way, the major dictionaries I’ve checked (大辞林 and 大辞泉) list two definitions for 今一つ:
- One more
- The state of being insufficient/incomplete
So that idea of incompleteness or something ‘remaining’ is present in 今一つ too. The Wisdom EN-JP Dictionary doesn’t have 今一つ listed separately, and instead refers readers to もう一つ, for which it gives the same definitions.
In 大辞林, both words are listed under one definition, with both pitch accents being listed as acceptable. The meaning for which a specific pitch accent is indicated is ‘more’ or ‘additionally’, and the pitch accent for that is , meaning one starts low, goes high, and stays high when connecting to the next word. For the other meanings, no particular pitch accent is indicated, though it’s true that in general, the pitch goes down (accent pattern , 頭高（あたまだか）＝’head high’) for the ‘already’ meaning. The point is though… I don’t think the two meanings are meant to be taken separately. 大辞泉, which is the dictionary made available by the Japanese search engine Goo, also lists the two definitions in the same entry, and even includes the word いま as part of the explanation under the ‘more’ definition. It may similarly interest you to know that one way which 大辞泉 explains the ‘already’ meaning is 「現に、ある事態に立ち至っているさま。」= ‘the condition of presently being in a particular state’. もう is not as clearly separated from the present – the ‘now’ – as one might assume. Also, there are more than two definitions listed, with the idea of ‘already’ being used to mean ‘about’ being listed under a separate definition (e.g. ‘it’s about to end’=‘the end is already here’. They’re not strictly equivalent in English, but they’re quite similar, and もう is one way of translating ‘about’ in Japanese.) My point is that the ‘more’ meaning of もう is in fact a result of its use to refer to what’s already true/about to be true in the present moment, which allows it to be used as a basis for comparison in order to indicate that something similar can be added. As 大辞林 says,
‘expresses the meaning that even though one has already reached a certain standard or state, one further adds [to it]. Additionally. [examples]’
They’re the same word. The two meanings are connected.
Here’s an example sentence from 大辞泉’s entry on もう: 「今泣いた烏がもう笑った」= ‘the crow that just cried has already laughed’. いま isn’t just confined to the present moment, and can also refer to the time period just around it, including the recent past. In this sentence, もう, strangely enough, refers to a more recent time period than 今, which is usually ‘now’. That’s an example of an additional nuance.
Simply put, everything I’ve just quoted from the dictionaries quite clearly shows that the line between もう and 今 is far from clear. The two words are used differently in general, but there are overlaps in the ways they are used and the time periods that they can be used to refer to. In any case, the fact that multiple major Japanese dictionaries explain 今一つ and 今一 using もう一つ, もう少し and あと少し should suggest that 今 does in fact have an additional nuance. As I suggested previously, I think the most intuitive way (for an English speaker) to understand this usage is to see phrases like 今一つ as verbal interjections of some sort: ‘[just] one [more] now’. In a sort of broken/expedited English, you might say ‘Now! One!’, and if you imagine a situation in which only one more effort is necessary (e.g. a whack-a-mole arcade game in which you’ve spotted the final mole that your friend needs to hit in order to beat the current high score), you might see why the Japanese expression is appropriate.
However, if you want evidence that 今 has the specific nuance we’re looking at right now, which it should share with もう, and to a lesser extent with あと (because like I said, I think the way one reaches that meaning for あと is through a logical extension of the idea behind ‘after’, whereas both もう and 今 encourage a comparison with the present, established state in order to discuss an ‘addition’), here’s the specific definition from 大辞林:
2. (adverb) The condition/state of further adding a little to a situation or state that already exists. Additionally. More. [examples]
Notice in particular that it lists さらに as a synonym, which is also the case for definition of もう in both 大辞林 and 大辞泉. More importantly, notice that もう is listed as a synonym for this particular meaning of 今. That proves that they share this nuance. Finally, note the fact that the language used by 大辞林and 大辞泉 is similar in the two definitions I’ve quoted even though one describes 今 and the other describes もう. Both discuss the idea of ‘further adding’ (さらに…加える) to an ‘already existent’ (既にある) ‘state’ (状態・基準・状況). They’re being used in exactly the same way here. 今 does have this nuance, and as I said, I believe that it’s very literally an expression of the idea that ‘one [more thing] in the present’ might make a difference.
The phrase I have in Japanese in my head is 「今、一つで、…」= ‘now, with one,…’, which isn’t even an example of the expression being used. To me, this is an example of how we might derive the meaning of the expression. If we knew that ‘right now’, using ‘one’ of something, we could accomplish or complete something, wouldn’t that mean that we’re aware that we need to add one more thing to the present state? Wouldn’t that then logically morph into the idea of incompleteness or insufficiency? This is just like how certain expressions in English morph into adjectives or adverbs even though they were originally verbal interjections or catchphrases. Think about sentences like ‘He’s a dime a dozen,’ or ‘It’s time to go all in.’ Consider the word ‘one-upmanship’. ‘A dime a dozen’ clearly isn’t an adjective, but it’s an observation of sorts that turned into an adjectival phrase. Similarly, ‘all in’ is a reference to games like poker, but the idea of staking everything on one thing has spread to other domains of everyday life as well. In British English, one can even speak of an ‘all-in fee’ at, say, a restaurant, in order to say that the fee in question includes everything that needs to be paid for, though this usage probably has nothing to do with gambling. Finally, it’s pretty clear in the last example that the idea of being ‘one [thing/point etc] up’ compared to someone else became a descriptor of sorts that allowed us to form the word ‘one-upmanship’. The same thing probably happened here, and I think this additional sense of 今 is quite a natural extension of what it originally meant based on how it can be used in phrases and various situations.