How come my fish finder doesnt show any fish when i stop driving?

Hi, i have a hummingbird 597 fish finder. I have the transducer mounted in the rear of the boat. When i start to drive in the middle of the lake a lot of fish show up, but when i stop at a point or near the shore nothing shows up and i know there is fish there. Why is this?
My transducer reads 40 degrees and 60 degrees. I can be in 20-30 foot water sitting there and nothing shows up.
I have the transucer mounted at the rear of the boat under the gas tanks

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