Diagnosis
The door hinges are misaligned or the latch strike plate is out of position. Over time, the weight of the door can cause it to sag slightly, making the latch and strike plate not meet cleanly.
How to Fix It
1) Open the door all the way. Look at the two hinge arms where they connect to the tub. See if any screws are loose. Tighten them. 2) Look at the metal strike plate on the door frame (the part the latch hooks onto). It's adjustable. Loosen its two screws slightly. 3) While the screws are loose, tap the plate slightly upward with a screwdriver handle. Retighten the screws. This small adjustment often makes the latch engage smoothly.
If That Didn't Work
Check the rubber door gasket around the entire tub opening. If a section is folded under, bulging, or has food debris stuck to it, it will create a seal too early and resist closing. Clean and straighten the gasket.
Heads Up
Don't force it or slam it repeatedly. You can crack the door liner or damage the latch mechanism, which is a much harder fix than a simple alignment.