At this stage I only have a basic understanding of ZigBee, but from what I've seen so far doesn't "slots" only refer to end devices? As far as I understand, a router device doesn't count towards any end device limits on the coordinator or any other router devices.
I now have six different router devices (XBEE, Salus and Aqara) with 28 end devices spread pretty evenly across them. The coordinator (HE hub) only has 3 end devices directly connected itself.
I believe that your sensor devices should be pairing as router devices and not end devices, so shouldn't be affected by any such end device limits?
Unfortunately I'm in the UK, so GE / CREE aren't devices that are available here.
I'm also not convinced that this would help given that other devices (Salus routers, XBEE routers, SmartThings end devices, Aqara end devices) have all paired without issues as can be seen from my previous messages.
The last Aqara end device that I paired I forced to pair directly to the HE hub by pairing it within 6" of the HE hub too and it was fine. The same test with your sensor devices didn't work.
We're suspecting that the sensors are being told to leave the network immediately or a very short time after pairing, but no other devices when paired are exhibiting this behavior.
Is there anything on the sensor side that would prevent pairing from fully completing in the first place? Perhaps some sort of timeout? Or is it expecting some data that it's not receiving from the coordinator?
Hopefully the ZigBee scanner will arrive soon and we can get to the bottom of it!