I think the higher you are the slower you need to be going to drop a torpedo. I did a test on torpedos (was a long time ago, may not still be valid) and the following situations worked.
Plane used: TBM
Speed: 450
Alt: less tham 50ft
Plane used: TBM
Speed: 300
Alt: 200ft
Plane used: TBM
Speed: 200
Alt: 400ft
Plane used: TBM
Speed: 150
Alt: 600ft
Plane used: B5N
Speed: 100
Alt: 1000ft
I think it was dependent on speed and alt. So the lower you are flying the faster you can drop.