I was having this issue with my PDI chamber in my VSR-10. I had the adjustment screws almost all the way out but the hop up would not go down past halfway through their range of adjustment. It turned out that the o-ring that holds the arms down had stretched from being removed numerous times for hop up cleanings and was not putting enough force to squeeze the bucking down through the barrel. I replaced the o-ring with a thin rubber band which I wrapped several times and voila tons of hop again. Sounds like you may have it figured out, but it's probably worth checking.