As they say, you have to have your coil over a target to find it. There just might not have been anything there to find from the war . . . . did you get any targets? While you said the farm was on the retreat path, have you confirmed that to be the case?
I lived in PA for a while and hunted the Gettysburg area extensively in the 1980's and again whenever the family went up to see relatives that are still there - in fact I was there the weekend of 10/19 and got to do some hunting in your area. There are some areas where one side of the road produces yet within a stones throw on the other side, nothing!
Atlanta was the same - a cleared lot did not always mean finds and sometimes the fields would produce halfway across then nothing.
If you were picking up targets even newer ones, you know the unit is working. Now you need to get the coil over some keepers. The Deus picked up a few 3 ringers last weekend for me in an area that was littered with square nails from a barn that was torn down in the 1940's and that was on a farm about 8 miles from the battle on the retreat path.
See if you can overlay your farm onto one of the period battle maps and confirm you were in a retreat path. Beware of the historical markers as they are often put where people can see them and not where the actual action took place (I learned that the hard way over the years)
Andy Sabisch