I have an SKS with the the tapco stock/mags. Trouble is, there are so many years of so many different countries making these things, specifications range quite a bit. All of my mags worked out of the box, but needed dremmel work to be fed by stripper clips and even then, sometimes they don't feed into the mag properly... I would like to get the original box mag working with the tapco stock one day. The box mag is way better than anything else, but I also like the tapco stock with the gas tube rail. Anyway that is aside from the point.
It sounds like your issue is that it won't feed/chamber off the mag. This happened to me a couple times. Make sure you're inserting the tapco mag in properly... it needs to go in on an angle with the notch latching into the right place on the stock before you push it up to click it in. I made the mistake of not doing so once or twice and the mag wouldn't feed properly. Heck I think I even got the mag stuck in there once... Get some dummy rounds and you can play with it at home. It will probably work once you master inserting it correctly. If not, it is probably the notch part that needs work.