Dragonfly 5 DVS locks on thermocline instead of bottom

[levistic]

Hi,
I bought a new Dragonfly 5 DVS last month from ca.binnacle.com to upgrade from my old black & white third party and I encountered some issues that I hope someone can help me with.
I fish in a lake having the max depth of 150 ft with a strong thermocline (school of baitfish?) at about 30-35 ft. At low trolling speeds the sonar locks on this thermocline (shown as a wide light blue, yellow and orange band) and the display shows its depth in the left upper corner instead of the bottom depth, which is much lower. I cannot zoom out to see any lower from that thermocline. Sometimes, when the bottom is not far below the thermocline, the bottom is shown correctly on the screen, but the displayed numeric value of the depth is still that of the thermocline.
Chapter 8.11 of the manual refers to manual settings of sensitivity for gain and noise, but I could not find them on my model, just the fish sensitivity and that one does not fix the problem.
Not seeing what is below the thermocline is a very serious shortcoming, in my opinion. This never happened on my old black & white third party fishfinder.
Please help !
Here are the specs of my device:
Product Info
Dragonfly 5 DVS
Product ID E70306
Product Serial number E70306 0650225
Hardware Revision 6
CPU Revision Unknown
Software Info
Primary Bootloader Version v14.05-00013
Secondary Bootloader Version v14.05-00013
Kernel Version v14.05-00013-00137-g86fa892 (3.2.0)
Platform Version v14.05-00013
Application Version v11.46-00305
Application build time Wed 22 Apr 2015 11:21:55
Media Version v13.03-00008
Thanks.