That's pretty deep stuff.
You need to make absolutely sure that when the boat is
in the water, and is fully loaded, that your transducer is
flat with the water, looking straight down.
If it's angled too far to front or back, I don't think it
will work well with the depths you are talking about.
Also, I'd put the sensitivity on max and keep it there.
Also, you didn't say, but are you just drifting when looking
at the depth. The faster you go in deep water, the less
chance you will get a good sonar reading.
Once you get the Humminbird transducer
dialed in and working, then I'd try
to switch to the Airmar. The Airmar may not
be 100% compatible with the Humminbird.
That's why to get the Humminbird transducer working
as good as you can before switching.
If the Airmar doesn't work well, then you know it's a compatibility
issue.
Also, if you use the Airmar, make dog-gone sure you are selecting
the right transducer type.
Good Luck.