Wondered why the connection betewwen my iPad 8th gen & 2017 iMac 4k using Sidecar (sets iPad as additional screen) wasn’t working, it worked OK on the 2017 MacBook Pro. So I tried it on the laptop & it wasn’t seen. But it had been visible in the past. Then I noticed that Bluetooth on the iPad was inactive. Turning that on got Sidecar working on the MacBook Pro but it was still cranky on the iMac, sometimes working via wi-fi, sometimes not & never via w sync cable. All the while, the wifi-only iPad Mini was only connecting via a cable.
I’m not conscrned for the Mini, it’s screen is too small to be viable as an iMac’s extra screen anyway.
In desperation I reset the bigger iPad’s network settings & when it rebooted after that process, it worked fine in Sidecar on the iMac, both over cable & wifi.
I had tried a soft reset earlier & it hadn’t helped at all.
It’s all good now but a 10.2-in screen is still a bit small for use with an iMac.
It’s viable besise the 13.3-in MacBook pro’s screen, though, providing a nice bit of extra screen realestate.