The increased back pressure (caused by a can) increased bolt speed. The bolt moveg so fast that the rest of the system can't keep up. Several things indicate excessive bolt speed. In mine, the indicator was that the follow on round was double-feeding, but not in the normal sense as it wasn't caused by a magazine failure. The (just fired) brass hadn't cleared the chamber by the time the bolt slammed home with a new round.
Brass not clearing the chamber is excessive bolt speed and the bolt needs to be slowed down. I put in a heavier spring and used a carrier weight to try and slow it down. No dice.....and I put a HEAVY system in that gun. There's just too much gas going back.
The only other thing is too much gas and an adjustable gas block is the way for that.

