the stem which connects to the front of the bolt is flared to help air efficiency. Depending what bolt you're using will determine the severity of the "flare".
Couple of things - you may need to try a different o-ring. They're rarely exact size and you may need to go a little smaller/larger to get the right fit. If you have several o-rings to choose from you'll be able to find the right one by sliding the top hat on the bolt several times and get a feel for the friction each creates. You want it smooth, not stiff.
Make sure you're using a healthy amount of lube. That's a major movement area and if its dry you'll lose that o-ring every time.
Check your regs - over-pressurizing will blow that o-ring