There are several circumstances where you can not castle:
1) If either your rook or king has moved
2) If you are in check, or castling would move your king through a square where it was checked, or if castling would put you in check
3) If there are still pieces in the way
4) Site technically if you are doing it the wrong way. You must click the king and then the target square.
Apologies for the terrible examples, but wanted to just demonstrate the basic principles without finding examples from my own games. If you're still confused, let me know and I will help
about every other game i have a problem with castling it won't happen-- so in most cases i have to give up -- i have run the review -- but it doesn't explain why i can't castle -- is there a reason i can't castle thereby losing so many games ----i would have lost those game anyways but my goal to to get to move 30 in every game -- not to win that's a improbity and something i have come to accept - but just to get to move 30 would be a accomplishment for me any help