[A. Smith]

Here is Ducimus' tutorial on how the AI works in SHIII and SHIV.

Here, I think, is your problem:

Originally Posted by The text linked above

There are several types of sonar, and they all differ in geometric statistcs. Each one becoming a bit more effective then the previous version. From all ive been able to tell, the AI will not ping you unless:

A.) You are in his active sonar cone.
b.) You are presenting him a favorable aspect
c.) You are doing both A, and B for X number of seconds.

Essentially, by putting yourself straight in the path of the convoy, you have all three conditions pput together.

What I find most effective is to put yourself maybe 6 or 7 kilometers (rougly that many thousand yards) and wait underwater with engines shut off; track them on sonar, and when they get close (say, 5 km) to the intercept point, start heading there a couple of knots faster then them. You should be able to get within a good firing range before the escorts spot you; when they do (that is, when they turn to head towards you) fire as quickly as possible and go deep. You should be able to escape.

When I say "good firing range", by the way, I mean maybe 2 or 3 kilometers. They won't see it coming any less if you fire from closer range, and they might even start zig-zagging - sometimes evading the torpedoes. Heck, sometimes the escorts won't even spot you from that close, so don't be afraid to fire then so you don't even have to be truly afraid of them.

EDIT: this is from my own limited experience running only RSRDC and OM (so no mods on enemy AI).