If you put more than 5 in the mag, does the gun still load the bullet into the chamber? If so then there seems to be a problem either with the mag sticking too far up into the bolt or the spring that fires the striker is too weak. Depending on how old the bolt is , it could be just a lot of residue built up under the firing pin. It can be slid out and the whole bolt cleaned with a spray cleaner like brake cleaner. i have 5 model 64s of different manufacture years and have not run into this problem with any of them. It really should not make a difference how many are in the mag unless the mag notch is so worn and the additional pressure upwards is binding the bolt or striker. Maybe you could try a new mag,.... may cure the problem. With out it in front of me it is very hard to diagonse.
