i think that the problem is that you probably were doing the hit-and-run style of fighting
eventually, when the skeleton is moved far enough away from its "original" place, it will become invincible to go back to the original place and regenerate hp
this should be a problem if you have bad connection and the game lags, therefore, ingame, the skeleton may have already moved back, but it shows that the sleletons "frozen" there
try tapping the skeleton itself, its what i do