Check your transducer placement. I have a similar setup and mine work
almost all the time. The small dash meters transducer is mounted on the
transom, and I mounted the garmin transducer in the bilge by making an
angled collar out of the top of a spray-paint can and putting the 'ducer
into it with some silicone glue. I guess your hull construction materials
will determine if a shoot-thru-the-hull method will work to your
satisfaction. Also, if your hull has chines that will funnel air bubbles
then you should avoid placing the transducer over those channels. The high
frequency sound has to pass through the plastic transducer, the mount
adhesive, the hull layers, and the water column......then back through them
all again to be received. Air is not a good conductor of this signal.
I have read about placing the transducer in an oil bath container, but don't
know if it works better. Most of the reccomendations I've read say to use
a harder epoxy-type mount, but I wanted to use silicone in case the bilge
location proved to be troublesome.
You would think that the manufacturers would have some side-by-side
comparison information on the efficacy of the various mounting methods and
transducer options.