As can be seen by other posts here, Apple’s CloudKit has a mind of its own about completing syncs. Until resolved, if all your devices on the same local network, perhaps add in the Bonjour sync method provided by DEVONthink. Normally more reliable and faster than Apple’s CloudKit anyway.
In addition to seeing instructions in the “DEVONthink Handbook”, also see Bonjour Simplified
No need to turn off Apple’s CloudKit while also having Bonjour. It might eventually work. But at least with Bonjour you can get all synced and get on with the more important stuff.