I bought a pair of GE/Jasco Zigbee plug in dimmers about a year ago; they refused to pair on my Hubitat C-3. Yet, both paired instantly on my SmartThings hub. C-3 had been (and still is) running fine on Zigbee ch. 18 and had never shown any Zigbee pairing issues with fifty or so other Zigbee devices aside from one other device that I never was able to pair (I had assumed it was defective). ST hub was on channel 24 as I recall (it has been a while).
As an experiment, I switched my HE hub's Zigbee channel to 24 from the Zigbee settings page and very quickly tried pairing one of the dimmers which I had reset per directions. It paired instantly. I then quickly changed the HE hub's channel back to 18 (I emphasise quickly because I didn't want the rest of the devices on that PAN to change their channel, as they normally would do. But even if they did, they should eventually change back). Within a few minutes, the newly paired dimmer on ch 24 automatically followed the channel change and was working on 18. I repeated the same procedure and got the second dimmer paired successfully on ch. 18 as well.
I can't come up with any reason why this would be necessary (or why it worked at all-- I would think that if a device wouldn't work during pairing on a certain channel, it wouldn't work in normal operation either) but it solved my problem and the devices have been fine since. No guarantees that it will work for you, but it's easy enough to try (fair warning: it wasn't disruptive to my network because I was able to perform the join and channel changes quickly enough; that may not necessarily be the case with yours if you try this. Worst case is you'd have some devices that might need to be rejoined if their operating channels get out of sync. Probably best not to do this if you're counting on all your automations working flawlessly during the process).
