When you wrote " the action closes" it does mean that it closes on EMPTY chamber, correct?
When you put a cartridge in SKS or AK's chamber, the rim portion of the cartridge do stick out in order for the extractor to grab it during the ejection process.
You told us that the cartridge refused to chamber whether you tried to do that via the magazine or by manually chambering the cartridge by hand. When you tried to chamber the cartridge via the magazine, did you " ride" the bolt or did you let the bolt run forward freely? If you "rode" your bolt, the action will not fully return to battery, i.e. the face of the extractor would sit at the base of the cartridge's rim. Since the cartridge's rim is not enclosed by the chamber, you may get the impression that the cartridge is not fully sitted / chambered.
All in all, I agree with others' suggestions: make sure your chamber is free of obstruction.
I've used all kinds of rounds from God knows what countries, and yes, I've fired thousands of UMC from three different SKS (all Chinese) and never have troubles with them. My friend, however, did have a slam fire with a UMC (soft primer). That is why you should purchase surpluse military ammos with hard primers when using your SKS.
Johannes