http://www.chess.com/livechess/game?id=1368189304
there's the game,I couldn't play dxc6 on move 18,I did the usual,after clicking d5 then c6 nothing, then dragging the d pawn onto c6 which usually works but not this time
After 17.. c5, it does look like you should have been able to play 18 dxc6 e.p..
I would write an email to support.
// Not that it is any consolation, but Stockfish thinks that dxc6 e.p. was your 3rd best move, after Bxd7 and Ng3.
http://www.chess.com/livechess/game?id=1368189304
there's the game,I couldn't play dxc6 on move 18,I did the usual,after clicking d5 then c6 nothing, then dragging the d pawn onto c6 which usually works but not this time