Rough weather?
Because if you've got big waves washing over the deck, the deck gun crew will abandon their positions leaving you all alone, which means that elevation works fine, but traverse doesn't.
You'll know you've got heavy seas if you see the "underwater" icon at the top flashing intermittently.
The "workaround" is to keep manning the deck gun, but you really shouldn't be trying to use it in heavy seas. It's not made for that purpose and you'll waste a lot of ammo that way. Hard enough to aim a deck gun in calm weather from a moving deck, well nigh impossible if the deck heaves up and down all the time.
