Yup, that was the story. The Trigger Maru does a great job replicating the issues with the mk.14 I've noticed. I think that's kind of cool, because it gives you a reason to consider taking the mk.10s instead, for the sake of reliability
One thing I noticed though when I started out my current career is that you could pick the S-boat and still arm it with mk.14 torpedos. Correct me if I'm wrong, but I believe the S-boat's tubes were to short to accomodate the mk.14?
Anyway, thanks for all your suggestions. I actually did manage to torpedo a merchant ship just now, so I might just have been really unlucky before