I suspect, but can’t think of a way to prove it, that it’s Apple CloudKit letting you down right now. There are other reports here and on other forums of Apple CloudKit not behaving as expected.
Before going to Dropbox, perhaps first step is to setup and use Bonjour. Requires that you use it on your local network, but it’s fast and reliable. Instructions to setup in the outstanding DEVONthink Manual and supplemented at Bonjour Simplified
I use only Bonjour. I’ve used Dropbox successfully in the past and can recommend it.