A) Batteries. Be sure that they are the correct quality and are working (voltage could be down) and inserted correctly.
B) Test the unit (see -How to test the trap)
C) Clean the metal kill plates (the two at the closed end of the trap)
D) You could have a fast Mouse – Put food at rear as normal plus 2 pieces in the middle of the trap Also turn the trap at a 90 degrees to the wall with front facing wall so the mouse must slow down and turn to go inside.