I had the same problem with a Chinese SKS several years ago. The firing pin froze in the out position. When I disassembled the bolt I found galling of the retaining pin into the firing pin which froze it out. I ended up replacing the entire bolt assembly and make sure it is kept lubed.
IMHO, this is a poor and dangerous design as the primer itself is used to push the firing pin back into battery before the hammer strikes. I have no idea why the Chinese never used a spring to return the firing pin into battery instead of a cartridge.