I think j7 is right. I had an Ithaca auto that did the same thing, and it turned out one of those pins had dropped out. The first time it happened I was in a duck blind with lots of birds flying, so I whittled a pin for it out of a nearby alder twig. It held up until I could find a replacement pin. Probably not the smartest thing I could have done, but there were too many birds around to go home!