Well, if the one barrel goes right in, and the other does not, and you've swapped them before with no issues, it's not the receiver, it's the barrel. There is probably just a few threads that got dinged on the barrel, and are making it hard to screw in. I have seen it on my own. Go over each one with a magnifying glass, and when you find the buggered ones, use a small (like 1/8" wide or so) triangle file to gently clean up the threads.