Are you using BlackThornProd's tutorial? Maybe you
a) forgot to assign the ground check in the inspector
b) forgot to make a new layer
c) didn't put the ground onto said new layer
d) didn't assign the new layer in the inspector
Here ya go. I actually fixed it by making the ground in one piece (before i duplicated the same ground sprite 4 times and it would say it's not on the ground randomly). It might be more time consuming to have to modify the sprite in paint.net every time i wanna change it but hey, it works so it should be good, right? ahah.