DWalker is spot on as far as testing the ring tension. Cam pins can wear, does yours have a ridge from where it has been riding?
If the bolt carrier cam slot is worn(unlikely)it will be quite obvious, it will appear peened.
Make sure the lugs in the barrel extension are in good shape and clean, I have seen chunks of a primer get in behind the lugs and prevent it from rotating properly, preventing lock up. A burr built up on them will do the same thing.
Make sure the gas tube is in good shape, not burred or bent and the gas key is properly tightened as well.