This is usually one of two things.
- It can be too much pressure being put on the trigger and can be fixed easily. To solve this issue simply unscrew the top screw of the handle by a tiny amount. This should release the pressure and allow the trigger to work again. See first image below.
- The grounding bar on the back side of the machine can become misaligned. Every time the machine moves into the back position it must come in contact with the bar. To fix simply rotate the bar back into it's original position and it should fix it. The original position is perpendicular to the machine. Watch video or or see second image below.