When I try to remember a kanji reading I think of words I already know with that kanji.
For Down (下) I thought of 下手. But instead of giving a warning it just counts it as wrong. The reading can also not be found on the page except for at the “Found In Vocabulary” block.
I assume it is a kun’yomi reading but I’m not sure. I hope this hasn’t been pointed out before, I couldn’t really find it on the forms.
That’s because it’s not considered a reading for 下
There are some words in Japanese where the combination as a whole has a special reading.
By that I mean that 下手 isn’t 下=へ, 手=た. It’s just 下手=へた.
You can verify this in dictionaries as well
Another example of this special “The combination as a whole” is 大人 being おとな. It’s not お＋とな, it’s just the whole thing is おとな.
Okay that makes sense, thanks for the explanation. 
The term for this is 熟字訓 (じゅくじくん) - it’s a Japanese-origin reading (i.e. 訓読み) which belongs to a kanji compound word (i.e. 熟字) and not to any of its components.
a tip may help for these two on’yumi reading: 上、下
i always remember the “joker” but in a japanese pronunciation of the word 〜joka 
上：じょう
下：か
Edit: now i think my reply have nothing to help since it’s a kun’yumi issue
