When the rounds don't eject and instead stay in the chamber (and sometimes cause a sticky bolt), the gas valve is often thought to be a culprit when it might be pitted and worn. Also, the gas tube itself can be at fault which would be my next guess if you replace the gas valve without success.
It is always nice to have another working SKS sitting beside the one you're troubleshooting for this issue, because you can sometimes interchange the parts until you find the culprit.
You should also crosspost your issue on
http://sksboards.com/ if you have not done so already.
CNCWarrior offers a
same-type newly-milled replacement valve. Murray, often seen around sksboards, offers a
different variant (no thumbscrew on the valve, instead mode is switched by a wrench). There may be other offerings, but I remember these two having come across them in the past.
As for a new gas tube if you need it, I unfortunately have not seen too many original issue floating around but of course tapco or whomever offers new tubes possibly with modern handguard for ~$50 (or used to).