Possible reasons below
1. Off-alignment of latch switches' You may have to adjust door and latch switches.
2. Open or loose connection of high voltage circuit especially magnetron filament circuit' Check high voltage component according to component test procedure and replace if it is defective.
4. Open or loose wiring of power relay
5. Defective Secondary latch switch
6. Defective power relay or Ass’y PCB [Replace PCB main if necessary].