How does it fire if you rapidly release the trigger? In that video you were going fairly slow, when I have my cockers timed for quick shooting they don't like to be played with like that. Snap in, Snap out. You can partially(totally?) cover the blowoff hole on the front QEV to see if anything changes but I doubt it. I will give this a try tomorrow, I am going through all my markers to get them ready for sale and I have a spare cocker and QEV here somewhere. I never had any issues with them before though....
Everyone says "Well maybe i'll make a difference",
Thinking they've got plenty of time
But you're squeezed a little tighter everyday
Finished before you can commit a crime