Question about HDS 7

[5th Tuition]

At slow speeds (1-12 mph) my hds 7 shows both bait/fish and depth of water. When I get up on plane; it still gives accurate depth but screen is blank of bait/fish. I tried putting the "ping" speed up to maximum; but it doesn't make a difference.
The transducer is mounted on the transom and it is flush with the bottom. I was told if I lowered the transom just a hair; that might fix it.
Any suggestions?