Could it be, that it was just hunted to death, by very qualified hunters, years ago? There's some machines that do a pretty good job around iron. And machines that can even get a coin under a nail or two, while still passing the iron.
I can think of a spot near me, that dated to the 1820s, that had a major "iron zone", where .... you litterally could not hear a clear/clean spot of ground with no iron or signal. When we first started working this site, we pulled lots of conductive targets from this mess (coins, buttons, cool whatzits, etc...), that were simply easier, or above, or louder than the iron. But as time went on, and we'd picked off all the easy gimmees, we had to go back through and listen more carefully. Then we opted for 2-filter machines, which separate and average better (although not known for going as deeply). Again, more stuff. Eventually even that dried up, so I even took a Compass 77b out to this zone, which can see through 3 or 4 nails sometimes, to get a conductive target (although it is a beast to operate, and tops out at 4" depth).
If someone were to work this site now, they'd be saying the same thing you're saying: "where's the coins?" or "where's ANY conductive targets?" And it might be confusing, because the ample iron indicates lots of human habitation, activity, etc....