Minn Kota battery monitor

Previous owner thought that it was only hooked up to 2 troolling batteries. I think he just did not want to admit that it did not work on third battery. When he showed me how to work it he stated that I could hook to cranking battery if I wanted to. When I went to do that I found that it was already hooked up but not registering for battery #3. I discovered that it was hooked up to all 3 but not monitoring the 3rd battery. Not sure if there is some reason or if it just not working correctly.