Haveing Fish Finder problems Please help

I have a Hummingbird 141c all that I could afford at the time it was on sale. I have the transducer pounted on my trolling motor. the trolling motor is shallow enough that I can see it if faily murky water. my problem is that most of the time when I try to read the depth in shallow water its may say that I'm in 4ft of water and I cant touch bottom with a 7' rod with my arm on the water. and it gives my a flase bottom looking reading and will not show to much of what I would think to be structure. can any one help. I have a fairly new motorguide digital seires 71 lb trolling motor