john -- if you are SURE your park is old enough to hold silver, then here's what I'd say...
UNLESS a few really good detectorists CLEANED OUT that park back in the late 60s and 70s, taking a good majority of the silver out...and now, the clad you are digging has been dropped more recently, in the 80s, 90s, and 2000s, then otherwise it is quite likely that the silver is in there, just deeper. If the park had major "dirt work" done after the "silver era," that could have put the silver especially deep. Otherwise, though, I would bet the silver is still there, but just simply deeper than you are digging. A park with that much clad tells me that it has not been hunted heavily, at LEAST not RECENTLY. So, unless it was hit HARD back in the 70s and maybe 80s, there will be silver there.
What's the deepest coin you commonly dig? I'm not talking about the deepest EVER for you, just your average "deep" coin...do you dig many coins over 4" deep? Over 6" deep? If you are digging 6" to 8" deep clad, then it's possible that coins sink fast in your particular type of dirt (or else, were buried by park groundskeepers with a layer of topsoil and/or sod laying). BUT -- if you are digging most of your clad in the 2-3-4-5" deep range, but not digging much of anything deeper than that, coin-wise, then you may simply need to learn to "hunt deeper." Your machine is capable of it, you may simply need to learn to set it up properly, and then listen to the more subtle signals (and not expect them to ID "perfectly").
Hope that helps a bit,
Steve