After getting けってい and にっこう wrong more times then I care to admit I decided to do some reading. I have no idea why I did not do this before and am simply posting this here incase someone else has the same issue.
The small tsu is called a sokuon and is used only before a kana that starts with “s” “t” “h” “k” and the “p” (voiced h). It cannot be used before any vowel kana or “n”.
This now explains why when you are counting most things you use いっ (いっき, いっさつ and いっぽ) but one machine is いちだい !!! Same for いちもんじ and いちねんせい.
Apparently there is some rare exceptions (Cramorant from Pokémon is ウッウ), but doing a search for っ does not show any times the above is broken.
I have never seen this rule in any of the text I have used to study. It is always just assumed you will remember where to use it.
Can anyone see why I would be wrong with saying this?