Before the patch, Frostbolts had this nasty tendency to prod the mob a bit. Just stop him in his tracks, for a split second, then move towards me again. Granted, I did see Frostbite procc from FB and work fine, but not 15% of the times. I never did a count or test, so I can't say for sure that little stutter of the mobs would account for the rest though.
After the patch, I did some finishing of quests I had left behind, and I didn't see that stutter anymore. Didn't really do any testing on it, but I can't say I saw much difference in the amount of Frostbite proccs though.
I'll be keeping an eye out on the forums though, for confirmation on whether or not it is fixed.