As far as transducers loosing accuracy it can be readily seen on ice fishing units where you can actually see your jig at several feet down.
Let us not confuse accuracy with sensitivity.
In sonar, accuracy would relate to depth. If the unit gives the depth as 21 ft. and the actual depth is 22 ft., the accuracy of the unit is ? 1 ft.
Depth is a time-based calculation. Unless compensated, water temperature and salinity will have an adverse affect on accuracy.
Sensitivity is the unit?s ability to detect objects in the water column.
Numerous things can affect sensitivity; however, the one that would explain your inability to see the ball (large frequency shift) would for the most part, render the unit inoperative.
Since you are not complaining about seeing fish, I suspect the transducer has moved. Cone angles are such that very little movement of the transducers will have a major affect on the viewing area. I would try raising the rear of the transducer up a bit and see if your down rigger ball comes back into view. If that does not work, I would suspect the integrity of the unit as a whole.
FYI: ice fishing units use transducers with much larger cones angles than used on a boat, thus the ability to see your jig closer to the surface