I was out last weekend taking photos of children in a park. The film back got stuck around frame #12-13 consistently. The film counter will not advance, but constant clicking on the shutter will eventually un-jam it. This did not always happen, but when it did, it was always around #12-13 frame. When I got home, I processed the film --- sure enough I had multi exposure on frame 12-13. Any idea what is causing this ? Thanks.