Sensitivity is not a constant because of different wind conditions, different oxygen content, different water temps, different water clarity from area to area. What is a good setting on one unit will not necessarily be a good setting on another one even if they are the same brand and model.
Put sens on manual and adjust it as needed to keep the clearest pic.
You may also benefit by trying different levels of noise rejection.
Changing upper and lower limits helps what you see as well as ping speed.
Rarely will one setting last all day long.