Pretty certain that your range to target was simply too close. As I recall, 500 yards for arming was required.
The Mark 10 was actually a sought after weapon once the unreliability of the Mark 14 became known. There was no meaningful defect ever reported with the Mark 10 firing pin and it was considered quite reliable. Firing angle should not be a suspect in this case - especially since you indicate that you lined up for a "straight shot" on a stationary target.
It actually would have helped if you had fired at a steep angle since the distance traveled to properly orient the torpedo angle and send it to the target would have counted toward the 500 yard requirement.
just one thing for you to do when you waste half of your load out for the rear tubes... and this is it:
