Are you sure you're using JUST Frostbite and Flames? And are you sure you're actually hitting the crystal?
(I know, those are stupid questions but a lot of the times personal error leads to a lot of these "bugs")
Since you've taken the time to create an account just to post about that bug, I assume you've already done all of that and it is still not acting properly.
Try to leave and re-enter the dungeon and if that fails then just find another filesave. Sorry but I haven't heard of that bug.