Most of the time the problem you discribe is related to the friction rings and the ammo used.
The A5 is a long recoil operated shotgun that depends on the proper balance of friction for the load being used. The friction is regulated by the positioning of the friction ring components, as well as by other factors involved in creating drag on the system. What I mean by that is cleanliness of the outside of the magazine tube, the amount of lubrication on the tube, and the amount of wear on the parts.
Obviously lighter loads require less friction, and heavy loads require more friction in order to make the gun cycle properly. Many times I have fixed the problem you discribed by applying 2 drops of oil to the outside of the magazione tube, and wiping them in with my finders to spread the lube. This of course reduces the friction .
Making sure the friction components are properly set up for the load being used is the first step. Cleaning and proper lube ranks right up there in importance to proper function.