Have you ever lubed/cleaned it?
Clean it well and lube it well. The rails and locking block should be wet with oil.
Also, as you said it was used, I would replace the recoil spring.
Also, can you tell if it was 1 mag or all of them? You may need a new mag or mag spring. Also, are these factory mags or aftermarket ones?
And, was this factory ammo or reloads?