I have a fairly new hb 788 that I seem to be having an issue with. While fishing a brush pile no matter what the depth is and marked with a bouy at times the pile disappears. Its not like Im not on the pile I am almost positive some of these are large piles and I stay positioned with the marker pretty good. Then sometimes magically the chart blows up with structure again. We even hung up in the brush while the sonar showed nothing. I have the terrova with us2, use 20 and 60 degree beam back and forth to see if it has something to do with it but I have no clue. Any help would be appreciated.