Why warship sonar can miss wreckage on the seabed

Why could Royal Navy sonar fail to find aircraft wreckage in relatively shallow water?

Warship sonar is optimized to detect moving or acoustically distinct targets, while silent wreckage on the bottom can blend into the seabed and require side-scan or penetrating survey sonar.

Different sonar systems are built for different search tasks. A warship set is designed around hostile vessels in the water column, whereas wreckage resting on or buried in the bottom can look like part of the seabed. The panel points toward side-scan and penetrating sonar as the tools better suited to separating a metallic object from surrounding sediment.