When in doubt, add more Zigbee Router devices and also do your best to actively avoid interference.
Recommend buying a few “known good” dedicated Zigbee Router devices when getting started with Zigbee so that those can act as the backbone of your Zigbee network mesh (being always available).
Zigbee signals are weak and very susceptible to RF noise / EMF interference so check out all these :
(above is my extended version of https://www.home-assistant.io/integrations/zha#zigbee-interference-avoidance-and-network-rangecoverage-optimization with more details and related discussions.
https://www.home-assistant.io/integrations/zha#defining-zigbee-channel-to-use
https://www.home-assistant.io/integrations/zha#using-router-devices-to-add-more-devices
https://www.home-assistant.io/integrations/zha#zigbee-network-visualization-in-zha-ui