How is your pan tension? Make sure it's not to light, so he has to put some weight on his foot to set it off. Which will ensure a higher catch on the foot.
With offset jaws, if your catching him by his toes, they can often pull out.
You might also try adding another swivel either at the point where the chain attaches to the trap, or where the chain attaches to the anchor.
Shock Springs work well for coyote also. 
