I have a few dozen surplus mags. A few arrived NOS, but most have seen some significant use. Only three of four of them are solid when inserted with no wiggle -- the rest can be moved back-and-forth and side-to-side a bit. All of them hold the bolt open and work without a problem. I did have a handful that wouldn't hold the bolt open. I took one of them apart and switched parts around with a good mag until I narrowed it down to the follower. I compared that follower to one that would hold the bolt open. Looking at it from the side, I saw that the follower was slightly "pinched" in. I pried it gently apart to match the good follower, and now it works. It was maybe 1, 1.5 mm difference.
As a lot of these parts came from different designs, years, runs, and factories, they may not match up perfectly, especially with an aftermarket receiver. If tweaking the followers a bit doesn't work, then your bolt catch may be just a titch short. You may want to add a little material with JB Weld and file it down to fit and function. While the JB Weld will be under stress when the follower comes up to engage the bolt catch, it should hold okay. If not then use the JB Weld to affix a small piece of metal and then file that to fit.
I know that some of the AR crowd put a strip of Velcro either on their magazines or in the receiver to tighten things up, although I don't know how well this would work with a VZ 58 type rifle.