Before even starting to troubleshoot any deeper you should make sure to read this and try to follow all the tips here → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
Regardless of what Zigbee gateway solution you are using, following all of the tips usually resolves most weird and intermittent issues (at least if the root cause was related to old firmware, EMF interference, and/or having too few Zigbee Router devices).