Why does the magnet controlled lock on my cat flap not always release like it should?

My cat wears a special magetic gizmo on her collar that is supposed to automatically unlock her cat flap when she gets close to it. I've noticed that sometimes it is a bit sensitive and will not unlock if the cat is not at just the right angle. I can live with that quirk; however, sometimes it will flat out not work at all. I've even taken one of the spare magnetic devices and waved it in all different directions, angles and proximities to the flap and it will still not release. Then, later on, it will work again. I am trying to train my cat to use the flap and it is very frustrating for both of us when the flap will not open like it is supposed to.