Sounds like interference. Have you tried changing the Zigbee channel?
I have four hubs at home, all on different channels.
- Hub#1's nearest bulb is about 3 ft away.
- Hub #2's is about 9ft with an intervening floor/ceiling.
- Hub #3's is about 25ft away and through an exterior wall,
- Hub #4s is about 35 ft away through an interior wall and floor/ceiling.
Everything's rock solid - apart from one very old bulb that occasionally appears offline in HomeKit but always works fine with the hue app.
Do you have anything else wireless in your house? Wireless home phone? I think wifi can interfere with zigbee on certain channels. Can you change your wi-fi?
Here's a good source:
https://community.home-assistant.io...-with-best-possible-range-and-coverage/515752