I am by no means an expert on Depth finders. I can say I have the exact problem with my 2 units. Pretty much like you I have 2 different brands, front unit is mounted on the trolling motor, and the rear is on the outside of the transom. Front unit will show fish, the rear one will show nothing. I have adjusted the sensitivity, and have set them about the same, with pretty much the same results. Front unit is about 3 years old, rear unit is brand new. Both have different names, but I'm pretty sure they are made by the same company. I am starting to believe the old saying, "Saturday Morning Cartoons"