According to an old Army manual for the SKS, it says this:
7. Replace recoil spring assembly:
a. Insert recoil spring assembly, curled end
first, into bolt carrier.
b. Pull receiver cover pin fully to the right.
C. Slide receiver cover into place from the
rear.
d. Secure cover with receiver cover pin.
e. Rotate pin down to locked position.
It is vague in 7a, but it pretty much says to lay it in there and not secure the spring into that shelf. Now, I'm not going to question those that own an SKS. Norm and papabear don't seem to have any problems with theirs.
In regards to the failure to fire, the manual says this:
Defect:
Fails to fire (cartridge in chamber).
Cause/Remedy:
Defective cartridge. - Reload
Bolt not fully locked. - Clean and Lubricate
I think you should just fire the crap out of it and see if you can get it to malfunction. If not, then the spring is okay how the gunsmith installed it. If not... then I don't know. You can always ask the folks on the SKS forum -
http://www.sksboards.com/smf/index.php Find SKS Troubleshooting down the list.