Diagnosis
The door hinge springs have lost tension or the door is out of alignment, preventing the latch from holding.
How to Fix It
Check the two torsion springs at the bottom of the door hinges. They should be tightly coiled. If they are stretched or detached, they must be replaced. Adjust the door alignment by loosening the mounting screws on the inner door panel where the hinges attach. Square the door to the tub frame and retighten.
If That Didn't Work
The door latch mechanism itself may be worn or broken. Inspect the plastic latch hook for cracks and the strike plate for wear. Replace the latch assembly if components are damaged.
Heads Up
Do not overtighten the door adjustment screws. This can strip the mounting holes in the inner door liner, requiring a full door replacement.