When they say, "I cast a spell," do you say, "Okay, cool. How?"
Because if you know what they're doing, how, with what -- invoking ancient gods, wielding eldritch energies, calling forth fell powers, etc. -- as well as the details of the situation they're in, then you'll have TONS of material to use on a 7-9. They're casting a magic spell! Just think of how cool, mysterious, and dangerous that is!
If you're drawing a blank, it's because the concrete fictional details of what they're doing aren't there, I'll wager.