Try varying your depth. Like Trop III said, if you are going for impact, make sure you are setting the depth to less than 4m. If you are going magnetic try setting the depth to about a meter less than the draft of the ship. The default depth of about 5-6m will result in you bouncing off the angled part of the boat.
imagine this is a ship with the m's being the waterline (assumes a 90 AoB)
Code:
x
mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
x
x <====== good, this will detonate, because it's flatter
x
x
x
x <====== bad, this might bounce, because of the slope of the keel.
x
<====== good, this will detonate if you have magnetic set when it goes under the ship.
That's been my experience at least.