One of the most common reasons for not detecting an officer at all is that he does not have his equipment actively transmitting.  Many radar and laser devices are in stand by mode where they are not emitting a frequency, so there is no signal for the detector to pick up and alert to.  Rest assured, if the officer is using radar, you will get an alert (assuming your detector is in good working order).  The good news is, of course, if he is not transmitting, he cannot capture your speed.

It's good to note whether you sometimes do pick up radar or whether you never do.  There are some sources of radar in the driving environment that will cause your detector to alert; stores with automatic doors, for example.  Even if you detector has locked out these alerts, you will see that it is still detecting them via the display.  This is a good sign that all is well.  However, if your detector never alerts at all, that is cause for concern and may require service to ensure all is working properly.  

Please be aware that other detectors running side by side can lead to performance issues including missed alerts and additional false alerts.  We do not recommend this practice.