You could be having the same issues I've been having with mine. I bought mine in Jan of last year. Initially it was leaving any other scanner in the dust in terms of sensitivity, esp 800MHz. By late March, it was acting like the attenuator was on constantly. Opened a ticket with GRE and they checked it out, said it was bad and sent me a replacement. Now, however, the replacement is starting to go downhill as well. Just opened a ticket with GRE yesterday, so we'll see what they do with it now. I checked the antenna screw issue, it was tight, I can't check the solder between the antenna feed and the PCB since it's under the shield, but I suspect that is the culprit.
Have you opened a support ticket with GRE yet?