Scan around in your yard a little and make sure there aren't any targets then got get a penny, nickel, dime, and quarter and lay them out on the ground and see what they sound like when swinging fast and slow across them. Also, hold the pinpoint button and make a couple passes in the east to west and then north to south directions and see where the coin is when you get the best sound, that way you'll know how to pinpoint them when you can't see them.
As far as them sounding different at different depths, that can happen. i have the 250 and have found that the ID can get a little jumpy the deeper the target is, I would assume this would be due to the amount of mineralized ground between your coil and the target. One thing that helped me was to scan the target one direction and then turn 90 degrees and scan it again and see if you get the same ID reading. Of course, dig everything for a while until you are comfortable with the machine.
Dudes