I don't think that there is such a thing as "too close". Maybe setting the Zigbee power a bit lower on your C-8 could help if you think that getting to close might also increase RF noise.
Things to look for:
- Make sure batteries still have some juice on them. I understand that the new IKEA devices really like rechargeable AAA at 1.2V (like their LADDA).
- Make sure you put the device in Zigbee pairing mode (4 clicks in 5 seconds) not Touchlink pairing (long press).
- Make sure you don't have any other hub on the same Zigbee channel (e.g., old C-7 still powered on, Hue hub, IKEA DIRIGERA, zigbee2mqtt, etc.).
- Make sure you don't have any other mains-powered Zigbee devices in close proximity (1 - 2 m) just to be sure that the RF signal from the Hub is the strongest so that the device is selecting the Hub as their parent.
- Check for interference with the Wi-Fi channel (but this would also disrupt other things too).
As a last resort, you may also want to play with Zigbee Pairing Helper to make sure that only the Hubitat hub is accepting join requests, but the procedure is overly complicated and hard to wrap your head around. You actually get the same results by just pairing really close to the Hub.
By 'really close' I mean something like this
Unfortunately I don't have a C-8 so I won't be able to help much more, maybe somebody else can help with their tips&tricks.
