Does the cocking handle move back at all? If it doesn't move, that's where your problem lays. Spray a good penetrating oil like Kroil or Liquid Wrench (WD40 won't cut it) into the bolt area and let it sit for a day. The cocking handle unlocks the breach block, which is locked into the barrel extension. Even send some oil down between the barrel and the receiver.
Another thing you can try is to remove the foreend wood to expose the friction rings. It's unlikely there is corrosion on the magazine tube holding things up, but check it's condition anyway. Now put the muzzle on a padded surface on the floor, and with the buttstock straight up in the air, treat it like a pogo stick and see if that frees up the barrel movement.