So the the scene with the Freefall and dragon are the only ones with water sprayers and the the dome is the only one with fog?
I know when Big Thunder reopened here in Disneyland they had issues with getting engough fog to build up in the tank for the next train, so some times they had only a bit of fog. I think they have fixed this by putting in a larger tank. Just throwing that out there.
There is also a blast of fog that is supposed to shoot up as you back away from the first voldemort screen.