Verify Receiver signal strength in the diagnostic page on the web GUI by going to the diagnostic tab in the side panel on the left. This view will show the signal strength from all Receivers. It should look something like this:
In the diagnostic plot you should see the signal from all 4 Receivers. The signals should be well above the noise floor.
Acoustic interference can be observed in the diagnostic plot. Echo sounders from your boat or nearby boats typically appears like burst noise.
If it is coming from your boat, try to turn off the echo sounder to improve position accuracy. In case of interference from other acoustic sources, use the GUI to try other channels to avoid the interference.