It is important to know that almost all wireless sensors have power saving settings inside the sensor to make battery live as long as possible.
Every time your sensor pick up an intrusion it transmit a trigger signal to your alarm, even if your alarm is not armed. This unnecessary wireless transmission thus uses unnecessary power. The power saving mode make the sensor send wireless signals less often when not necessary. You can set your sensor to send a signal every time it detects an intrusion (best way to test your detector) or to wait 5 seconds, 50 seconds or 5 minutes between transmissions. If you, as an example, set the detector on 50 seconds it will wait 50 seconds before it will send another signal.
Setting the setting to 5 minutes will have the best battery saving option but it can seem as if the sensor do not work because of the long time delay.