I don't have the PC version. In the PS version however...
I don't exactly remember what devouring was like but your description sounds pretty reasonable. The 'Miss' is basically saying you haven't managed to devour. I think this works the same as Quina's 'Eat' in FF9. You have to pummel your opponent down to a certain fraction of their original HP. Only then will you be allowed to devour.
I don't know correct me if I'm wrong.