"disappearing signals" is a common lament from beginners. And there's simply no way for anyone to diagnose it from printed text. It would be like asking someone to "please describe the sound of C-major in printed text". It can't be done. It has to be heard. So too is it with md'ing related sound-issues.
Every time I've corresponded with persons who have this issue, and ended up meeting them in person to help out, it always boils down to silly simple explanations (like creature 850's example, BUT DON'T GET LOST IN THE EXAMPLE ! ) .
Another example is the fellow was using a fast motion machine (XLT or whatever), and attempted to slow down and stop over the target each time he'd hear a beep. And since it was a motion machine, and he was slowing his motion (in an attempt to "hear it better"), the target would disappear. Doh ! And when I asked him : "Didn't you read the instruction where it said 'motion required' ?" He said : Yes, but I thought that simply meant moving the coil side-to-side as you walk. And I thought : ''That's a silly instruction. How ELSE do they expect anyone to walk around md'ing, if they're not moving the coil side to side. I mean, duh, do they expect someone to stand there with the coil motionless ? You'd never cover any ground". So he just dismissed that as a silly non-sensical instruction. Without realizing what it meant.
Do you see how no amount of printed instruction can convey that ? You have to see it, hear it, etc....
Another time a buddy was getting a "disappearing signal" at the corner of an old army barracks building. But no matter how big and deep he dug his hole, the signal was deeper and deeper and kept beeping ! After 10 aggravating minutes of enlarging and deepening his hole, he finally called me over to scan it. In 5 seconds I diagnosed it : It was a metal flashing/ribbing on the corner of the building. And he wasn't aware that the edge of the coil was also sensitive. So he was getting a signal from the leading edge of his coil, that he mistakenly thought must be deeper.
So my advice is: Hook up with a proficient hunter, and trade off flagged signals. Watch the signals he'd chase, verses flutters he would ignore. Watch the way he isolated the signal. Watch the way he digs, etc.....