Assuming torpedo failures, etc. aren't part of the problem:
1. Determine target speed. Doing so via the bow wake and/or smoke works fine, as does matching it's speed per derstosstrupp's tip.
2. Position your boat to intercept the target at a 90 degree angle. Google "Fast 90 targeting" for details.
3. Position your boat to be 3000 meters or less from the above intercept point.
4. Sink the target
It really is that simple.