Cobra Electronics ESD 6100 Radar Detector

I bought The 6100 at K-mart for half price. It's original Price was about $100 and I paid $50. So I thought it was a good buy because it had all the features I was looking for: 360 degree detection, all bands, and it was small. X band seem to give me good warning when I came upon a state trooper on the first day of the purchase. Then, however, when I was in my hometown, a rural community, I approached a city cop using k band. It didn't alarm until I was 10 or 15 feet from his vehicle, and I know his radar was on the whole time because a friend of mine has a whistler radar detector ( a cheaper model I might add) and his went off blocks away from the cop. I like everything about this radar detector, the lights, the sounds, the features, everything except for the fact that is just isn't affective.