Legal moves question

Hello all, I decided to learn the game of chess (literally yesterday) and downloaded this app to learn and play. I feel I understand the basic legal moves that each piece can do but I keep running into issues where I can't see why I cant legally make a certain move. For example I'll run into situations where my pawn cant advance forward and diagonally to take out another pawn. I don't see why I can sometimes and other times cant. Another example is sometimes I cant move my rook at all. I have open spaces next to the rook's space but cant move. I've tried googling lists of legal moves and I'm not seeing any rule that I'm breaking. Can someone point me in the direction of where I can learn all the special situations where I cant move? Or is it possible the app is glitching and not allowing me to do things I should be able to do? Thanks in advance

It could be a bug but you should also make sure your king is not in check.

Edit: or that the pawn or rook in question is not pinned. 

Your pawn was probably pinned to the king.