A general rule is this:
If you try to do a move that doesn't work and it leaves pink squares behind - then it's an illegal move that the system doesn't allow you to do.
Try to find out why the system doesn't allow you to do the move you want:
If is it a castling, then read about the castling move in detail.
If it is a en passant move, then read about the e.p. move in detail.
If you cannot move the king, see if the square is in check.
If you cannot move another piece, see if your king is in check, or will be, in check. Etc.
If you have doesn't all these checkings and still don't understand why the system allow you to do the move, then use the Help forum. Tell which game it is, and what move you want to do.
If you cannot get any help from there, then send a feedback to the game moderator. Use the link in the bottom of each page.
However, this takes unneccesary time. Try to find out the answer yourself first.
(Russ, if this advice is good, then consider move this posting to the FAQ or help file. There are many questions of this kind and much frustration during the time.)