My detector set on auto everything shows thermals clearly.  When I cross a hump that comes above the thermal I see the bottom come up through the thermal and then go back down through the thermal and as the bottom gets a few feet below thermal it disappears and the detector shows the thermal as the bottom.  No fish show up below a thermal.  When I was fighting a big laker I could see him just above the thermal then he would take another 10-15 feet of line and no longer show up on the detector. When enough line had been brought back in he would show on the detector again.  So my question for the detector experts is what parameter do I change to get my detector to see through thermals?