I own a shop, and the only thing I can think of is; you are right with everything you are saying. There should in fact be some kind of groove to retain the clamp, or it will just pull right off. On the matter of the lift kit; yes the lift changes the angle, and would in fact make the boot come off more easily. You can do this; Get a Dremel Tool with a carbide stone in it, and grind a groove around the hub since it's slick, and this will give it something to keep it in place. As long as you grind it to where there is a lip to retain the clamp, then it will stay in place, if not; it will keep pulling off.
The boot is full of grease, and I would love to see anyone that could put one on without getting grease on the hub too. Boots are made of polypropylene, and are stiff. Sorry, they don't make any that are soft and will flex beyond so many degrees. Try a 4x4 supply store, they may have a longer boot that will do the trick. When a boot is stretched beyond its capacity, it will pull off, and I don't care what its made of. A store that specializes in lift kits & 4x4 parts, more than likely has a solution for you. Grinding on the hub is not the best of ideas, but who knows? It sounds like the boot is too short to begin with, and when you hit a bump it pulls right off because its only made to stretch so far.
Glad to help out, Good Luck!!!