The Black Star Dragon Balls were reactivated when Kami and Piccolo reunited. This would mean that each time people went on a dragon ball hunt, the radar should have detected 7 dragon balls right next to each other, yet this did not happen. Why is this? Definitely just a plot hole due to GT being made as an after thought. Perhaps though, this could be explained and made so that it can somehow make sense.
EDIT: I am well aware that the Black Star Balls are scattered across the galaxy when used. However, I am reffering to the times they searched for the normal dragon balls before the cell games, during the majin buu arc, and then when they wished for everyone to forget about buu. At this time, the Black Balls were not used yet. So, the radar on earth should have detected 7 balls in close proximity on kami's lookout.