It can be a number of reasons but before you troubleshoot any deeper you first need to take actions to read and understand the best practices for avoiding EMI/RMI/EMF interference + make sure to add Zigbee Router devices (repeaters/extenders) to have enough coverage and extend the distance, because Zigbee is extremely sensitive to EMI/RMI/EMF interference and have very poor radio propagation (bad signal penetration of building materials and very short range), as if you do not do that before then troubleshooting will be much harder if not impossible. Try to follow all the tips in this guide: