Occasionally Keychain sync will stop working on my Mac, usually it's easy to kick into gear again (it's gotten a lot better than it used to be).
I think right now the only thing I can't for the life of me get working is syncing Text Shortcuts to my MacBook Pro. Everything else (seems to be) fine.
Universal copy and paste is also a bit of a black magic there, but I don't really use it enough to care (and where to begin even troubleshooting that?)
Oh, iTunes Home Sharing stopped working between my Watch and both my Apple TVs too (which is what the Watch Remote app uses), I could still see my iTunes library from both ATVs, but my Watch couldn't see either one. Signing out and back into iTunes Home Sharing on both Apple TVs resolved the issue.