Where exactly are they having problems? The thing is, that thaumaturgy can virtually do anything, so if it's only a small part of it that causes problems, it might be a good thing to locate that first.
A lot of the problem, I think, is them not remembering how it works each time it comes up. When it comes time to do a Thaumaturgy spell, I keep having to explain to them the relationship between their Lore and the Complexity and what and how much needs to be done to build up the spell; then how to go about the actual casting and how their Conviction scores relate to that bit. They're fairly clear by this point that involves rolling Discipline, but I've found trouble getting the idea of what the rest of it means to stick with them.
It's the mechanics that trip them up, mostly. They've got a fair grasp of the kinds of things it can do in the fluff--lasting veils and divinations they've got the hang of on account of them being fairly low powered spells they can cast more or less like Evocation, and they have a good idea of where the line is for 'no, you can't do this with evocation, so it has to be a ritual of some kind.'
So they know roughly what they can do with it, it's just the mechanical
how that keeps tripping them up.
