Re-measure
In most cases, it is sufficient to re-calibrate the door contact again, provided that it is mounted correctly. To do this, simply press and hold the S button until the LED indicates the appropriate mode.
Green = window/door with tilted position, open, tilted and closed.
Blue = window or door only open and closed
Red = window or door only closed and open, but inverted (when contact is closed open is sent and vice versa)
Range
The HomePilot could also be outside the radio range of the window/door contact. Mounting the window/door contact in aluminum window frames is not always recommended. Aluminum can shield devices that communicate via radio. In these cases, another 230V DuoFern device near the door contact can help as a repeater.
Ambient temperature
Another possibility would be that the window/door contact is exposed to an ambient temperature below 5°C, causing the battery to become very weak.
Battery
Possibly the battery is empty and the door contact no longer has its actual transmitting power. Replace the battery if necessary.