Using Calibre to allow for shared highlights/notes on multiple devices?

[iseroma]

Hi. I hope I can make my question clear.

I use Calibre and have 12 Kindles and an Android phone running the Kindle App.

This morning, I noticed that if I use the "Personal Documents" page on Amazon's "Manage Your Kindle," I could send The New Republic, fetched by Calibre, to my different devices. Better yet, if I highlighted or wrote notes, they were shared with the other device. It was like my Calibre-fetched New Republic had become an Amazon-supported document.

This got me thinking. Would it be possible to do the same thing with a Calibre book that is not on my Amazon account? So I sent one of my Calibre books to my Kindle, and it showed up on my Amazon page as a personal document. So far, so good. Then I sent the book to my phone, and it appeared on my Kindle App. But then I noticed that highlights and notes did not sync between the devices. Upon investigation, I noticed that the Kindle copy of the book had sync option grayed out.

A few questions:
1. Why would I be able to sync highlights and notes on a fetched Calibre magazine/newspaper but not on a book?

2. Am I doing something wrong?

3. Any ideas to allow notes and highlights to be shared on Calibre (non-Amazon) books on multiple Kindles?

Thank you very much for your help.