Sonar not showing information.

[Petros13]

Hi all.
Have a problem with an Interphase forward looking sonar.
When I set it to operate in either the forward scan or the down scan mode, the visuals on the screen appear to operate normally, but no water depth reading is shown. Current depth in the marina is 10 to 12 meters, so it's not out of its range. Forward scan mode shows that it's scanning ok both forward and down.
Any suggestions on what may be causing this anomaly?
Thanks in advance for any forthcoming thoughts. Peter