You might check for galling of the locking lugs. Some years ago I had a fellow bring me a REM 700 the was exhibiting exactly what you are describing. He was a "clean" freak and was afraid to leave ANY lube on anything. After several shots he could barely open the bolt because of the galling, fortunately just on the locking lugs. I was able to clean them up and convince him that a wee bit of lube was a good thing.
It could also be the cocking piece dragging on the ramp of the bolt but since the other bolt was better but still a bit rough I doubt that is the issue.
I know nothing about these specific actions so the next thought might be out in left field. Is it possible that a heavy duty striker spring was installed in the bolt? I have installed 30 lb. springs in some 98s at customer request and it definitely adds a bit to the power required to cock the gun, I assume the the 640 is a 98 clone of one sort or another.