I agree with earlier suggestions that you’re probably holding the forend to the rear when firing.
Note the little tab along the front of the triggerguard. This is the action bar release. When the shotgun is cocked, one end of it projects from the triggerguard while the other end (inside the receiver) blocks one of the action bars and prevents the bolt from coming rearward.
When you pull the trigger, the action bar release pivots out of the way so you can cycle the action. Pull the trigger on an unloaded gun and you will see the tab move.
If you apply pressure against the forend when you pull the trigger, the action bar holds the action bar release in place - it cannot move and get out of the way like it supposed to. If you release the pressure before trying to cycle, it will move out of the way. If you don’t, it won’t!
If your shogun jams up again like this, push the forend forward, then try again to cycle the action.
If this is your problem, you might want to remove the forend, bolt, etc. and use a flashlight to check the end of the release on the inside and make sure you haven’t damaged anything when forcefully cycling the action. (I’d avoid removing the triggerguard assembly, if possible. While not difficult to do, reassembly can be a pain in the unlikely event that one of the shell stops jumps out.)
Incidentally, I don’t believe this is short-stroking. Short-stroking is failing to cycle the action all the way to the rear before moving it forward again. Generally this results in a failure to feed a new round, in more extreme cases it can also result in a failure to completely eject the spend round (usually resulting in a stove pipe situation).