Is it possible you're trying to do en passant when the rules don't permit it? If you are on the 5th rank--say, e5, and your opponent moves his pawn to f5, you can take the pawn as if it were on f6. That is, you simply move the e5 pawn to f6 and take the pawn that's on f5.
But you may only do this immediately. If you don't take en passant on your immediate next turn, you forfeit your right to do so.
Hope this helps.
I am unable to do en passe. please help. For some reason it is not working. Is there something special I need to do?