This is an issue with duolingo program. If you put in an answer that is only wrong by one letter then it considers it only a typo and won't mark it as wrong. Sometimes it tells you "you have a typo" or "watch the accents" or something similar, but sometimes it glitches and just let's it through without saying anything.
This happens a lot in Gaidhlig because there are a lot of words only off by one letter. "Tha/Cha" for example. Also he and she (e/i), and sometimes even for adding the 'h' when leniting words (so it would accept "seanair" when it really wants "sheanair", for example)
Unfortunately there is nothing the volunteers can do about this. The developers could reprogram the app but that would mean taking away the typo feature which is useful across many languages