There are many reasons why your radar detector may not have detected the radar. One simple reason is that your device is not set up to detect the specific band or type of device the officer was using. For example, a radar detector will not tell you if the officer is using a stopwatch or LIDAR to check you speed. Another possible explanation is that modern traffic radars only need to radiate a signal for an extremely short period of time to check your speed and your radar detector may not have picked this up.