if you clean or brush too hard the stuff falls down especially if it has not been painted too much.
I would try a fox tail cleaner or something like this that would get into the valleys and clean them and also being gently. Sometimes those angular brooms with the feathered edges are gentle enough to clean the popcorn with out damage. This especially happens around ceiling vents.
Once cleaned, be sure and change your AC filter often and use a good pleated one.
A good coat of sprayed paint on popcorn acts like glue or sealant and keeps the fall out to a minimum and it does a better job the more you paint it.