Ice and Fire - Rage Eater does not work in boss fights?

[EM1388]

I had something similar in a non boss battle after one of the first uses of the rage eater. I was getting some 2s and some 0s instead of 5s. Not sure why. My last two battles it has worked perfectly though and I didn't pay attention in a few prior uses. I know that is not much help, but I will look closer when I use in the future to see if it's a bug or some unknown trigger.