It is my understanding that Simonov designed the SKS with a spring-loaded firing pin but the arsenal people thought it was too complicated and substituted a cone-shaped pin going into a cone-shaped hole -- which can cause the firing pin to lock even if it isn't dirty. Anyway a spring-loaded firing pin upgrade is available.

I have the same model SKS but mine's been bouncing around loose and is much more beat up than yours.

Willard