if the BB falls out of the barrel when you point down, that means there is absolutely no hop up on. If this still happens when your hop up is turned high, then something is wrong with your hop up.
Alot of things can cause this. Torn hop up rubber, missing hop up nubbing, improper assembly etc.
Just buy a Prometheus Soft Hop up sleeve(It includes the nubbings as well) and install it. ICS is extremely easy. You just pop the upper receiver and take out the hopup/inner barrel assembly.