I've been noticing similar problems with dar. The other day, I had 6 contacts over 15,000 feet near me, only 1 contact was showing up on radar with a dot, he kept disappearing and reappearing from Radar [EDIT:] well within the dar circle, and none of the other contacts were on dar. They were all a good 5-8K inside the dar ring, over 15,000 feet in altitude, and we had friendly dots and dar functioning, so our HQ wasn't down at the time...