I have one experience i can relate. I was hunting with CTX and 6 inch coil, i got a nice tone but on target trace it was looking like a big piece of iron. I worked it from several angles and just couldn't make myself dig it up. About a week later i was eating a burger right across the street from this same lot and i had my cz3d in the truck with the small coil. I thought i would go over there where i had that signal and check out the cz on it. I got good signal with it and bad almost instantly from every angle. I was about to walk off when i decided to pinpoint and just see how big a piece of iron it was. It was big, but right at edge volume dropped off only to rise again about 1/2 inch away. I played around and could determine two distinct objects. I dug the plug and at 6 inches i got a nice merc. Was happy i went back. When i pinpointed with the ctx it was just a huge iron trace blob and i could not differentiate two distinct targets , although ctx did have good tone. I figured it was just false on the edge of iron. I have just never been able to bring myself to sell my cz's , they are extremely good machines for dated tech. I know this was not a cz5 but still a cz.
Anyway...how long have you had your machines for? Are you thinking things are getting missed for some reason?