Is the U touching the trap body when it is set? After a certain number of firings the U will get so bent that it touches, and then the trap will not release well. This is a major design defect which I reported to them and got no reply. Anyway I suggest looking at that first. It could also be birds taking the bait, I have big problems with that. A third problem can be that the trigger pin / latch contact point can get a bit misaligned; I have had to use pliers to squeeze the latch (the piece of metal you put the trigger pin on) to make it stick a bit further up.
Both the tube and Kania traps require some work to keep in top shape, for me its a lot more on the tubes due to this bending U issue. I am more a squirrelinator fan now, so far I don’t see any flaws in their design other than the fox going after them. There is in fact a coyote in our neighborhood and I am wondering if that is not what went after my trap. Foxes are small, I don’t know how they would move a big trap 50 feet.