Check the rods for straighness or breaks. I had one sticky pop on my Black Hole that had a perfectly clean break on the rod on one side. The break was almost imperceptible when it was fully up, but often the bumper would bind coming up because of the asymmetrical force on it. New ring and rod assembly fixed it. However, that fix required disassembling the whole pop bumper, so I ended up rebuilding the whole pop in the process.