"Do you want more?"
Translation:An bhfuil níos mó uait?
Ar mhaith leat tuilleadh is what we would commonly use to ask for more but it wasn't accepted
tuilleadh definitely should be accepted, but I'm not sure about the ar mhaith leat. While it has the same meaning, Duolingo's notorious about wanting the word-for-word translation (with ar mhaith leat being "would you like").
Though, I agree - ar mhaith leat tuilleadh sounds more native (heck, just using tuilleadh instead of níos mó)