Oh, that. That's a property of ALL spells. It seems Radium forgot to mention that, I think because it was also a rule in the 2nd battle engine. But, anyway, it's kind of common sense that you aren't allowed to use hypnotise over and over- if you could it would be far, far to powerful. Plus, you had already seen the water elemental spell go away when you cast it. Did you think that was a special property only owned by summons?