DIAGNOSIS The hinge mechanism or the door latch assembly is likely worn or out of alignment. These doors get opened and closed thousands of times, and the plastic components can develop slop.
FIX First, check for obvious debris or food particles around the door seal and in the latch hook on the oven body. Clean it out. Then, inspect the three hinge points (two on the bottom, one guide on top) for cracks. Gently try to re-seat the door by lifting slightly and closing.
IF THAT DIDN'T WORK The primary latch switch, located inside the front panel where the door hook engages, might be failing. A faulty switch won't give a positive 'click' feel. Replacing it requires removing the outer cabinet.
HEADS UP A door that doesn't latch securely is a safety issue. The microwave has interlock switches that prevent operation if the door is open. If it's loose, those switches might not engage properly, so don't use it until it's fixed.