Sometimes the inner tube moves, How fragile is the inner tube and is it healthy to push it up so the joint fully seals with the HT?
Yes, you should push it up so it makes a seal (when you put the HT on, it should not touch the metal that has "HOT" printed on it, but be a milimeter or some above it being held by the male joint). Of course do it when cold, so the glass is smaller (less friction) and less likely to burn you or crack or anything. Or maybe the surrounding heater will expand more when hot, so there would be less friction when hot, but I recommend to try pushing it when cold first.
The tube is held there only by friction, it's not fixed so it can be pushed with enough force in both directions.
The friction should be high, it shouldn't move easily, some Clouds will have a lot of friction so you can't move it at all without fear of breaking, and some less so you can push it with some force.
After you push it up, don't use force on the HT so it doesn't get pushed back down.
That's all can remember VX said a while ago about this.