Did you first drop the transponder overboard to get a baseline test and then try one of the three follwing in hull check methods? Norcross and KK's thread recommend either trying (1) trandsponder in plastic bag filled with water, (2) in bilge with enough water to cover transponder, or (3) vaseline coating on transponder then to hull? These help the unit conduct properly with no air bubbles which should allow you to find the sweet spot a bit easier.
Recommended in hull locations by manuf and others here tend to be on starboard side of keel, just fore of the jet pumps or bilge. Sorry if this is a dupe to your efforts.