I think what may be happening are these new radars are “seeing things” incorrectly that could be mistaken for a UFO. That means the signal processing needs a lot of tweaking.
In fact, things were seen visually that were not seen on radar. For example, in the Nimitz sightings, Cmdr Fravor saw the "tic-tac" hovering over the 737 sized craft in the water. That craft in the water was never picked up by radar (at least such has not been released by the Navy as yet).
As an aside, Fravor recently made the retrospective observation that it is possible that the tic-tac's behavior when it noticed Fravor was a distraction (i.e. when it saw Fravor's aircraft and came up to meet it, the craft in the water disappeared as Fravor's attention was distracted by the tic-tac).