Loc Chau.

On the top of the mag there is a little dimple. Press it down with the tip of a cartridge or small screwdriver. With it pressed down, slide off the top plate. It will only slide off one way, it's tapered.

Clean it out and oil it. My best guess is that the outside case is pushed in and its binding the follower. Check and be sure you can pull the follower up and down in the case by the spring without binding.

Another thing to check is the overall length of the cartridge. The ZB39 was made for a 8x56R which has a length of 75.91 mm. The 7.62x54R is 77.16mm. The ZB39 mags I have have ample clearance for the longer 54R round. Possibly your 54R is over length.

Joe