Sounds like two possible flaws:
1. A soldering joint is touching the case if you close it.
This can be repaired by resoldering the joint thats probably too "high" or just use a bit duck-tape to make sure it doesn't "touch" the case anymore.
2. When you close the back somewhere on the PCB a connection comes lose these can also be soldered but finding the faulty connection is a bit more difficult, you have to check all the connections that go to the Magnetic reed-switch and resolder them with your soldering-iron.