Check to see what security level the devices are at. One big difference between now and a year or so ago is that now devices which support S2 security may join with S2 security enabled, and at that point they can only be associated to other devices with S2 security enabled.
Here’s the thread with the recent discussion that @philh30 mentioned. The topic title is a clickable link. It’s a long conversation, because he ended up trying a lot of different things before finding something that worked. But it’s possible that your issue might be one of those earlier things so I would start from the beginning of those posts.