It could be poor machining in the receiver or left over crud inside.
Take barreled action out of the stock and using a small flashlight like the tiny flexible neck ones, check inside the receiver. Sometimes there will be left over cosmolene and debris stuck inside the lug recess area.
The last 91/30 I got from a seller had small bits of fiber stuck inside. I ended up soaking it down with Break Free, blasting as much out with compressed air as I could, and digging the rest out using a small steel pick set, kinda like the ones dentists use. I pulled out strands of what looked like bits of cleaning rags.
Make sure the inside of the bolt is clean too.
If you do all that and still have a hard time with the bolt, run your finger around the inside of the chamber and check for burrs or sharp edges. Sometimes if I get a Mosin with poor machining inside, I'll take a shotgun brush on a section of cleaning rod, mount it in a cordless drill, and using some lapping compound (or even oil) and low speed, I'll scrub out the inside of the chamber. Just be sure to flush it well afterwards with cleaner and oil it well.
For more info on tough Mosin Bolts check out Surplusrifle.com's section on 91/30 rifles. There is info on the right side of the page.