So the Hornet radar was severaly bugged last patch but is working great right now. (Well as it's designed by ED) Most likely you aren't adjusting the radar elevation to where it can see the target. You adjust the antenna angle with the HOTAS command "antenna elevation control." You can see the maximum and minimum altitude search values on the yellow TDC cursor on the attack format. The top number shows the maximum view of the radar cone at the cursors range, and the bottom shows the minimum value.