There can be several reasons why a perfectly functioning radar detector does not provide an adequate alert:
The police car did not have an active radar/laser gun
In order to alert, the police car must have a radar or laser gun, and the gun must be turned on. Just because a police car is on the side of the road does not mean it has a radar gun. And if you see a radar gun, it might not be turned on at the moment.
Police did not use radar/laser to clock you
The police may be using a non-radar method of clocking speed, such as VASCAR. With VASCAR, an airplane or helicopter measures the time it takes a vehicle to pass 2 markers on the road. A timer is started when you pass the first marker, and then the timer is stopped when you reach the second marker. Your speed can be calculated based on the time it takes to travel between the markers. The pilot then radios the information to a police car. Since no radar or laser is involved with VASCAR, it cannot be detected by a radar detector.
Incorrect installation
The radar detector needs a clear, unobstructed view down the road. The detector should be mounted at the windshield, and there should be no obstructions (such as windshield wipers) in front of the detector. The detector also needs to be level. This is accomplished by bending the mounting bracket as needed until the detector is level. For best results against laser detection, mount the detector as low as possible on the windshield. Do not mount the detector behind any window tint, otherwise laser detection will be severely reduced.
Issues with the windshield
Your vehicle may have an electric defroster built into the windshield or a special coating on the windshield. Both of these items can block radar and/or laser signals. To check whether your vehicle has any of these, please inquire at your auto dealer or the auto manufacturer.
Police used "instant-on" radar to clock you
With instant-on radar, the officer waits until your vehicle is very close and then he turns on his radar gun. Your detector will not alert until you are right on top of them. The best way to detect instant-on radar is to detect the officer clocking someone ahead of you. If you are the only car on the road, be careful!
Issues with the local terrain
The officer may have been positioned over a hill or around a curve. Curves and hills will reduce radar range. Use caution when approaching curves and hills.
Intelli-Mute was turned on
On detectors with Intelli-Mute, the Intelli-Mute may have been engaged. This can result in only 1 beep which may have gone unnoticed. Try turning Intelli-Mute off.