this is a question for lowrance, but i will give you my own personal experiences if you want to try. if your boat sits, down in the transom very far, then, the mounting of the transducer needs to be on a small angle to allow for that. it needs to be mounted so it is parallel with the bottom once one the boat how it sits in the water.
this may help your problem.
i do get bottom readings while moving slow. my bottom readings start to go about 1/2 throttle. almost all of my finders in the past that has been the case. lowrance may have a solution to that, i just havent gotten that far yet. i have , however, been very happy with the down on the structure scan staying on the screen at full speed. good trade off! i split the screen with the sonar and structure scan