I did the same thing a couple days ago. I noticed about the same results. Tried different modes, All Metal and Coin/jewelry and noticed only a little improvement in All metal. Forget a dime at 8" no way. I am in Ct. and there is iron in my soil. I did have a penny buried at 7" in my old test spot with mainly topsoil in it, and it picked it up at 7" but did have a little double signal in C/Jewelry mode. I was using the Platypus coil in all the testing. I did find a piece of 3/4"x 1/4" aluminum ring and got a reading of 37 with it at 7" in the iron type ground. It varied in tone and numbers though, and was a bit hard to lock on. I have a nickel at 7" and I could just barely read it and if I didn't know it was there, I surely would miss it. I was using the external speaker as it was pretty quiet outside. I tried changing the sensitivity from Auto to upper numbers, but found the sensitivity set to above 16 was giving way to many eratic signals. It seemed to work best in Auto and I was using it in low trash, set to F mode. This was in the C/J mode. It was close to the same when sensitivity was at 12. I am going to put on the Sunray 12" coil and give it another shot.
Ok just did it with Sunray 12". In All Metal with Sensitivity at 12 set to F mode, low trash, I could read all the items fairly easily. Numbers did jump around a bit though, except on the 6" penny, they were pretty stable. Going up beyond 15 and including Auto sens., it lost a couple of the items, (and was about the same as the Platypus in reading depth when in auto sens). Went to C/J mode with other settings the same and lost the 8" dime and real thin and weak on 7" nickel where I would likely miss it in normal hunting. Found if you go too slow you get nothing half the time and it will ignore most items below 6", except some after a small but quick reading, but when you return, it is gone.. I find that quite annoying and if you wanted to hunt pretty slow, it would really hurt performance. I know this is normal, but it wasn't as bad on most other detectors I have used. I also find it pretty sad that you have to hunt in All Metal to get decent depth. You buy a fancy machine with all these great features and have to go to the old dig everything mode to get real performance. I almost feel I might as well just use a pulse detector, except the sounds are too closely defined unless you buy the newest ones out. Now this is all in dirt digging. I hope to find better results when I get to the beach soon.