Kmrere42: what did your misfire primers look like?
Was the primer strike mark a light dimple or a 'square hit'. A light dimple means a light strike, or the ammo has a really hard primer. A square hit means a proper firing pin strike, but poor ignition most likely due to age or poor quality ammo.
As 30-30 man said, the SKSs have a floating firing pin and it should rattle when you have the bolt out, if not, your SKS bolt needs another cleaning. You could also have a short firing pin, although that would probably give you misfires all the time with a very light primer dimple.
I'm not familiar with that brand of Russian ammo but some of the Russian brands come out with some very hard primers and that could be one of the problems. HTH. Mikey.