One more thing to check. MOVE to a different location.
True story. I got to one of my Michigan lakes and got all suited up with my tank and other gear. Tried the detector and it falsed like crazy. Turned it off and on a few times still no go. Took my gear off, changed the batteries and still the same problem. I hit it a few times and I was really upset. I was beside myself thinking my CZ-21 was broke. I decided to go to the lake and play with it anyway thinking I might be able to get a signal anyway. Got in the water and my machine worked fine. Three hours after my dive I got out and was walking to my truck. A beautiful Afghan dog was in the yard. It hit me then. I was checking my machine OVER an underground dog fence! I really was frustrated before the dive. I just laughed going back to my truck. No sidewalks in the neighborhood on that side. Their fence went close to the road.