I dont understand

That may be because he does not have his radar unit activated. Radar units work by sending out a microwave signal. It bounces off your car and back to the unit. It then calculates the whole distance over time thing and figures out your speed...it's more complicated than that, but I don't feel like giving a class...and I'm not a real genius anyway when it comes to physics and whatever other -ics are involved with the science of radar. Point is...radar units have a hold function..kinda like turning that microwave beam on and off...when the officer has the radar on "hold" there is no beam coming out..which means, there is nothing for your radar detector to detect. When he takes it off hold to clock your speed, the magic beam shoots out, bounces off your car, and returns to the unit..then tells the officer your speed...by the time your radar detector actually lets you know you're being clocked, Mr. Officer is already turning around to come pull you over. Hence the reason radar detectors are a waste of money...but then again, there are plenty of cops that ride around with the the sending out a signal all day and never actually put it on hold...if a cop is using the radar properly, the radar detector in your car will not help you...but, not everyone does what they are supposed to do, so therefore, it may save you from one or two tickets.
Now, the reason it doesn't alert you when the police are right in front of you is because either the police in front of you; A)do not have a radar, B)do not have their radar on, C)they have it on "hold" and are not transmitting a signal, or D)your radar detector is broken and you need to get it fixed or replaced.