Also, some things, even from bosses, are not 100% steals, you can get one or the other of something. But for the most part, what Mirage said holds true. For example in the (SPOILER)Gilgamesh battle, you can steal one thing at first, then at a certain hp % you'll get a cutscene, after the cutscene there's something new available, and so forth. Sometimes the actions aren't as flashy, but the mob will take on a new character, casting different magics, or attacking in a different manner.