Alarm contacts are sensors which use a magnet to close a circuit. When the magnet moves away from the contact, the circuit opens. The distance the magnet moves before the circuit opens is called the "Sensor Gap" . The gap is affected by the distance between the contact and magnet, orientation of the magnets poles to the contacts internal reed switch, the material which the contact and magnet are mounted too and size of the magnet.
Improper selection and(or) installation on any of these factors can result in a small gap, inconsistent operation and even complete failure over time do to position changes of the door, window or whatever the contact is protecting. Too small a gap can cause false alarms from excess door movement during winds. Wood doors can warp over time resulting in a failure of the contact to close.
Before leaving any installation you should check the sensor gap on doors and windows. Activate the CHIME feature of the burglar system and slowly open each door/window. If wireless, allow time for the wireless transmitters response delay. A good gap is usually when the door is barley open. A large gap may seem preferable but can backfire on you. Open the door until the systems shows an open door. Close the door just to the point where it can move on its own without force. If the door shows closed, this may cause a problem. If someone leaves the door un-latched (open), but the doors shows closed, the alarm system can be armed. All it takes is a gust of wind or a pet pushing the door open to cause an alarm.
Guides addressing the installation of contacts: