The soil is mostly rich and sandy. After about 6 inches it turns to thick gooey clay. I sincerely doubt anyone has ever hunted this area. It is about 200 acres of the thickest briars and underbrush you can imagine except for the old road leading ferry dock and that immediate area. There are a few clearings here and there where the town site was supposed to have been but otherwise it appears to have been uninhabited for many many many years.
May I try to help? I've made a list of a few possible causes in this post before but I don't know if any of them helped. Let me try a few suggestions here and let me know if any sound feasible to you.
1.) Since this is a ferry location, (water), do a little research into possible floods in that area over the years. You mentioned that there's about 6 inches of sandy soil over a clay base. I don't know a lot about soils but that seems kinda shallow to me. Almost as if there have been regular floods to wash away top soil over the years. My point? Maybe the finds are further downstream than you figured. Or, gone altogether?
2.) If you're sure the detectors are acting correctly, try taking the access road and hunt backwards toward the main road. Start near the ferry and work back up one side of the road to see what sort of finds appear. I'm thinking that at some point the types of finds will change. When you get past the point of any flood plain.
3.) Bury a few examples in the soil so you know what they are and see how your detector acts.
4.) Take a screen next time and scrape a few yards up to see what actually shows up in the screen. If nothing shows, then see number 1.

5.) If there are any trees in the area, Try hunting very close around the bases. Sometimes the roots will hold things, even in a washout.
Most of all, don't let one failed spot spoil it all for you. Take a break from trying to force it to give up something you just KNOW has to be there. Try going somewhere that is more of a "normal" spot. A known school/church/park/etc. If you've had success somewhere, go there for a bit. The key is to find SOMETHING! ANYTHING! Even a day with a lot of clad quarters will put a smile on your face. I gar-ron-tee!
Then, the next time you do try your researched spot, go with an open mind and not expecting anything. Dig everything for a bit and see what shows up.
Also, try researching another spot for a change.
I sympathize with you and am very interested in seeing how this plays out for you. Keep us advised of your updates.
Good luck and HH!