Seconded this, with an additional bit of information. I dug up a hornet's nest/beehive near the main building, which counted as a treasure. Subsequent digs at points indicated by the metal detector counted up "treasures found" but no items spawned for me after the hive.
However, this was after several attempts at each point, since I was still figuring out just how to interpret the signals from the detector. Wondering if maybe too many dirt balls and holes is somehow interfering with spawning the items.