The first thing to do is clean the heck out of the chamber. Don't use bore solvent. Use brake cleaner, acetone, or denatured alcohol. Build-up of lacquer in the chamber is the most common culprit with these. Turning up the gas will eventually result in ripping the head off of a sticky case, leaving the front portion stuck in the chamber (and you don't want that). After you have cleaned the chamber, turn the gas down to 1.2 and work back up until it functions reliably. The less gas, the better.
Another thing to check is the fit of the gas piston. If it is too loose, excessive gas blow-by can contribute to poor functioning. The piston (the part that screws into the gas valve) can be expanded a little by clamping it in a vice, placing a ball bearing in the end, and striking the bearing with a hammer. Just go easy and try the fit until it slides, but doesn't have excessive play.
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