Examine removed primers (wearing gloves) to see if the decapping pin is piercing the primer cases. If it is, the primer body is stuck on the pin shaft and just gets drawn back to the primer pocket when the press ram retracts. I think that would indicate that the pin is hitting the primer with too much force. Slowing down to a press instead of slamming motion should help. It should also indicate that the pin needs to be blunted rather than sharpened.
I have never had this problem with a Lee decapping pin but immediately experienced it when I bought a Hornady die for one of my calibers. All I have done so far is slow down to a "press" motion rather than a "ram" motion. It is easier on me too.