Sometimes drain/psych just doesn't work. I'm not sure what the formula for the spell is (I'm sure it involves the magic stat), but I've seen it happen in a few FF games.
Even if it says some hp got taken off, but the actual hp didn't decrease?
I know sometimes it says 0, but I'm not referring to that, I mean, it said, for example, 15, and yet 15 didn't go down on my hp.