There are only two possible causes...
It's either a hardware adjustment or an electronic setting adjustment that is needed.
As far as hardware, the transducer needs to be mounted perfectly according to the manufacturer's instructions. For most, that means that it must hang slightly below the hull and be perfectly square with the water. It also must be tight enough so that it does not flip up under higher speeds.
Assuming that your hardware looks to be in order, your next step is to adjust the unit itself. On many units, the gain needs to be increased when the boat is moving. Try that first, and then get back to us if the problem is still there.