Updgrade the firmware on the Zigee Coordinator of you have not already.
Also try re-pairing (without removing) far away Zigbee End devices that did not connect via a Zigbee Router device.
If you done all that and still have issue the. might have remove some Zigbee End Devices and add Zigbee Router devices first before adding other devices.
In addition recommend get some ”known good” Zigbee Router devices, again read and follow this → Guide for Zigbee interference avoidance and network range/coverage optimization