Im new to the SKS and had exactly the same problem with a Tapco stock and Tapco mags (chinese SKS). I had bought 2 mags, one had a black insert (the part that the bullets push down on) the other had a beige one. The black one worked fine and beige one didnt. I bought 2 more from Accuracy plus, both beige and both didnt work. Comparing the 2 I found the problem was one tiny part at the top of the mag lock hook was too thick and was preventing the bolt lock from dropping down after the round was fired. it would lock the bolt back each time. I sanded the 3 mags with beige followers (or whatever they are called) to match the black one and they all work fine now.
Hard to describe, easy to do... sorry I can't be of more help. Theres nothing wrong with your gun or the mags... they just need a little sanding.