There's really only a couple of reasons this would happen. First is what I talked about in my first post. The bolt locking pin is getting stuck some how and you working the bolt back and forth gets it to retract enough for the bolt to finally move. Check to make sure that this pin is moving freely. There is small roll pin that holds it in place. Check to make sure that it hasn't worked loose over time and isn't binding.
Second is that you have a very dirty, rusted, pitted chamber but you say that you clean this rifle after every hunt so I doubt the chamber is screwed up.
Third is that your ammo is way over pressure but you would easily see signs of this on your brass.
I guess it is possible, though not very likely at all, that your firing pin is broken and is allowing the bolt head key pin to move slightly off center and bind up the bolt.