If you're using GWX 2.0 or 2.1, you need to enable the "Late War Sensors" mod. This fixes the stock bug in SH3 that affects radar and sonar in u-boats from 1943-1945.
Using GWX 2.1 as an exmaple:
In your enabled mods list, it goes in first....
Late War Sensors
Merged Campaigns
AOTD-AA-Ship-Schleuse-Bugfix
Community Units
etc...
Hope this helps. I've noticed several others having this same issue.
RTFM.