Trap takes some strength to set and requires excavating enough soil for the jaws to operate freely, but enough soil under the trigger for the mole to push it up.  Our soil has quite a bit of clay and if it gets between the jaws, the trap will not close.  Also, moles abandon a tunnel if it is disturbed too much or dig around the trap.  While it works on occasion and doesn't require as much disruption of a tunnel as those "step on" traps at Lowes, I prefer the plunger traps over the jaws traps.