I am not an expert in fridges or magnets but...
Something on one of the shelves or a salad crisper box is jutting out and hitting the door - stopping it from closing.
Or
Something on the door is jutting out and hitting against a shelf.
Door closure magnets on fridges are relatively weak - you can see how weak with the flat blade of a knife - any stronger and it would make opening the door very difficult.
Manufacturers would not fit fridge doors with magnets that go-off in time.
The magnet will be fine - the fridge or something in it will be causing your problem.
Good luck...
Peccavi, August 2013