Shoot 'em too hot and the lugs set-back creating a hump. When you open the bolt you have a little bit of springback that allows the bolt to start moving.
When the lugs try to leave the "valley" created by the setback it is an uphill run and the force required to get past it can be heavy.
I assume the bolt is not difficult to operate empty?
Has there been a history of hot loading?
Look at the bolt lugs and look for incipient cracks angling down and away from them on the bolt body.
My father liked mausers and loved to load stuff hot, so I have seen more than a few such cracks!
