I'm still having a bit of a problem understanding your question. But perhaps that machine has what's called a signal boost (or various terminology to that effect). What that means, is that some machines amplify ALL signals to equal loudness. As opposed to shallow ones sounding "louder" and deeper ones being "whispers". The manufacturers who implemented this on some machines over the years did so with the notion that this would help persons to "hear the whispers". So there's no more graduating loud vs whisper distinctions. All signals sound the same. So in order to know depth, all you can do is look at the depth meter, or cross-check in all-metal pinpoint mode. Or raise the coil to see when the signal stops, etc...
Most md'rs found this very annoying. Because you would loose all sense of deep vs shallow. Everything start to sound the same.
I am thinking this is what you're referring to, since you allude to how the signal is "still strong" even to the 7 or 8" level. But then ... presto, it just rejects or disappears the moment you reach a certain depth. Right ? So that just means you have a given magical point at which it's "loud" versus when it "disappears" . There's no middle ground. I find that VERY ANNOYING.
If so, then see if there's a way to turn off your signal boost (or whatever the B.B. pro might call it). So that you will have a graduating weaker and weaker signal, as you go from 1" to 3", to 5", to 7", and so forth. You will still loose the target at a certain depth ("rejects" or "disappears" or whatever you want to call it).
have I understood you correctly ?