No, on coin-sized objects I have found the depth meter to be pretty accurate. On trash and larger items, it's not too accurate. You say you're flat on the ground...that's item one. Obviously if you aren't, that equates to a higher depth reading. And on the shallow targets, 6 to 8" and less, I agree with the depth indicator shading all the way in. On the deeper ones, like some silver dimes, indians, and buttons I've dug...I've seen it be barely a stain on the bottom. You have to have a deep target for that and it doesn't sound like you've had any.
Why is that? Well, my first guess would be sweep speed. If you're used to hunting with another machine that can be hunted fast with, you need to adjust. To get the deep targets, you need to be very methodical in your sweeps and patient. If you're finding older clad coins, wheaties, etc. then you really need to slow down because the silver is there. Second thing would be the sensitivity. If the area is clean you should be able to run as high as 17 or 18. In fields, the woods, and at the beach I run 18 all the time. AT parks, schools, and trashier areas then 17 or maybe 16. Other than that, I can't think of anything else. I doubt there's anything wrong with the machine, but...you say "the last few times I've been out", so does that mean it WAS working better/differently? If so, I guess only you would know. In that case, it might be the machine. You could send it in and find out I guess. I did that with a DFX I had once. It seemed to me that it had suddenly lost all of it's depth. It went from finding a Barber dime at 8.5" to not digging anything over 3 or 4" EVER. I sent it in to them and received it back "Meets all specifications". Not what I wanted to hear.
