Are all the parts original?
You had it completely disassembled?
You lubricated the bolt, etc, as you reinstalled them?
I have had the bolt get bound up before, especially if it wasn't original to the gun.
My only thought would be to go backwards, disassembling until the point it frees up.
At the point you're at it should be pretty easy to remove the coupler and see what that tells you.