Hey Dennis,
Here is my interpretation of your Problem: Aluminum is reading like a coin, so you dig and find aluminum. You are only finding clad coins and no old stuff.
Turn your MANUAL sensitivity down in trashy areas. Keep coins with you in the field so you can compare signal pitch to mystery targets. Make little sweeps within the main sweep while in trashy areas. Don't use the crosshairs to identify targets (I have found barber dimes where the crosshairs register iron).
It is easier for me to use sound instead of the screen. If I am looking ONLY for silver coins and I get a big nice coin hit sound that sounds like a shallow hit (big strong beep), I get out a wheat penny and see what the penny sounds like. If the mystery signal sounds lower than the penny, I do not dig it. I assume it is aluminum pulltab and do not dig. This method is not %100 effective because large aluminum items such as screw caps sound high like a penny. But this method will screen out most smaller aluminum items.
By the way, if you go by sound only, you will be able to make a more accurrate decision on target composition and depth rather than if you use the screen. I never use the screen to decide on target composition and depth--ever.
If you are only finding clad coins and no deeper old stuff, you may be operating in a trashy area with your sensitivity turned up too high, thus not getting very good depth. Your threshhold presence will indicate how low you need to be turning down your manual sensitivity setting. If I am in an area where there are so many targets that my threshold does not return intermittantly in between SOME of the targets, then I will lower my threshold until it does return intermittantly. I don't go below 15 usually, but sometimes I may have to lower it to 10. Lowering sensitivity reduces the area of influence that shallow trash targets have. This helps you to see inbetween the targets a little more. Also when I am in a real trashy area (targets are about 4 inches away from each other, I double sweep 4 inch distances along my main direction of sweep (like little sweeps within a sweep).
If you are worried you are not getting the proper depth out of your machine, do an air test where there is no interference by ground targets or electric equipment. Place a silver quarter on the ground an see how high you can wiggle your coil over the quarter and still get a response. It should be about 9 or 10 inches at a manual sensitivity of 27. If it is, your machine is doing just fine.
Mike