Tabbycat;
Have you checked to see if the pan is dropping all the way and hanging so the jaw can't completely close. I have had this happen on a certain brand of trap a couple of times. The pan is sometimes a little large and if it goes completely down, it won't let the jaws snap completely shut leaving one jaw loose. If this is happening, I just grind off the side of the pan a little and it will work fine then. I like the brand of trap and will continue to use them, but just every once in a while, I will find one like I described.