that has happened to me too,
i think that could be because of some reasons,
1 - your internet conection
2- your browser (i recommend Mozilla Firefox)
3- some (temporary) problem on chess.com
remember that the last week chess.com had some changes, but everything goes ok. i 've done some problems on tactis trainer without any issue.
=D
I really like the tactics trainer, however the last couple of problems stopped and wouldn't register my move and the timer kept on going. That's okay, perhaps it got tired! I will try again another time. Thank you chess.com. This is a great site!!