Typical causes and steps to resolve false alarms from a door/window sensor can be found here:
I recommend checking to make sure the magnet is lined up properly, as some magnets are aligned with the top or bottom of the sensor instead of the center, and that the magnet is on the correct side of the sensor. The sensor should have a line or arrow on the side, which should line up with a similar line or arrow on the magnet when installed correctly.
Some sensors may still function with a misaligned magnet or with the magnet on the incorrect side, but it will reduce the maximum gap you can have, and the sensor may start to have issues as time goes on.
If everything looks to be installed correctly and the recommended steps in the guide don’t resolve the issue, then the sensor is likely experiencing a hardware failure and would need to be replaced. The recommended replacement sensors for your system can be found here: