It sounds like your Cointax is balanced too negative, maybe because the previous owner "slewed" the ground balance. To set the ground balance, hold the mode switch in the momentary position, to the left unless someone flipped the switch, and tap the button on the right under where it says GND OFFSET. Bob the coil as you would setting ground balance on a manual ground balanced detectors, keep holding the mode switch in the momentary position and tapping the button until you get a slight increase in the threshold when the coil is a couple of inches from the ground. Release the mode switch and that's it. The 2.2 chip remembers the settings, so it should still be balanced the same the next time you use it.
The Cointrax doesn't have a conventional manual ground balance, but using the "Slew" feature does the same thing as manually balancing with a knob on other detectors with manual balance.