My money is on the 'lifter/striker', commonly referred to as the transfer bar.
Montveil, I think the rest of the stuff they did was just because they had to tear it apart, so they just throw the kitchen sink at at.
BTW, you wont 'know' if it is fixed until you get enough rounds through it to be confident.
Oh, and it is TOTALLY BOGUS to have to pay return shipping !!!