Had this problem myself while playing the Med. campaign for WDAD.
The cause (in that instance) was discussed in this post
http://www.subsim.com/radioroom/show...&postcount=135
Until it's resolved here, You
can still use the sonar by placing the cursor into the repeater and spinning the mouse wheel. The bearing indicator won't move, but the bearings on the notepad will change and the green lamp will light-up when you find a contact - which you'll also hear if strong enough.