When the o-ring is the problem, it'll oven hang a spent shell out the side without fully ejecting it (short stroke). So that doesn't sound like your issue.
Drop the whole trigger out and give it a good cleaning. My guess is it's going to be the feed latch gummed up. Sounds like you've checked everything else already.
It is far more important to be able to hit the target than it is to haggle over who makes a weapon or who pulls a trigger.
— Dwight D. Eisenhower