Difficulty with frozen bread dough
I've had better luck freezing bread dough before it rises, or at least right after punching it down.  At normal household freezer temperatures, the yeast may keep growing very slowly.  If you freeze the already-risen bread, you risk getting a yeasty-tasting flat pancake :-). 

And of course you're right to look at the defrosting process too.  If you try to push this too fast, you could end up killing the yeast entirely.