It's working as it should. What might be confusing you is that if the sight is aimed where the turret can't fire - that lone turret wont fire.
So say you're aiming down in the front sighting station and you press the single fire - the top front turret won't fire. But if you press 'fire all" - the bottom turrets will fire where your sight is pointing which is lower than the firing range of the top front turret.
Basically there are situations in all of the sighting stations where if you press single fire, the turret you are controlling wont fire due to it's firing range / collision with the airplane.
So if sight your target, press all fire, those turrets that have line of sight with your target will fire.