If liner is too tight it could cause receiver to ride up. Or if too much air builds up inside the liner it will cause it to ride up and eventually pop off if air is not bled off by pushing the button. The one way valve in the end cap is designed to bleed off excess air and not permit air to come in through the valve (one way valve). If this valve gets clogged, it cannot do its job and air builds up with no way out, resulting in ride up and pop off. Make sure valve is functioning, that the ball bearing inside is freely moving (you should hear it rattle when you shake the end cap). Rinse it underwater thoroughly after each session and make sure valve is clear. The other possibility is that the receiver is not sealing properly causing air leakage which exceeds the valves ability expel it. If the valve is constantly letting air out during your session, you may need to tune up the liner to get a better seal, or even get a notch smaller liner.