Diagnosis
The door latch mechanism is misaligned or the plastic hook on the door is cracked or worn.
How to Fix It
Inspect the white plastic latch hook on the door itself for cracks. Then, look at the metal latch catch inside the unit's frame. Ensure it moves freely and springs back. Clean any grease or debris from the catch with a cotton swab and alcohol.
If That Didn't Work
The latch solenoid that pulls the catch closed may be weak or failing. This requires disassembly to access the solenoid on the main board.
Heads Up
Do not use the unit if the door won't latch. The safety interlock will prevent heating, and forcing it can break the mechanism completely.