Thought to mention one 'edge case' that I have encountered:
Manage Content and Devices shows 'Home screen' collections if you have ever had a Fire tablet and created any. These will not show up on a Kindle.
Not sure this got clarified but removing a book from the device does not remove it from collections. If you open the collection on your Kindle you can toggle to ALL it should show it in the resultant list.
When I want to download a book that's not in my Kindle, I find it is usually more efficient to use Search: just start typing the title or author name, and it will give you a few results that you can tap on. No need to explore page after page of listings. And of course you can Send it from the Manage Content & Devices page.
Depending on how large your Library is and number of Collections, it can take awhile to sync. Even on my Fire HD8, theoretically a much faster device, I remember it took more than a few minutes and perhaps better part of an hour after a Reset I did. Kindles might take longer.
On my Kindle Voyage, adding a single book to a collection takes the better part of a minute. I have about 1800 books, 8 collections.
I don't actually Download any collections on my Kindle. If I want to browse one (not often), I pick ALL and set view to Collections.
I reset my Oasis kindle 3 days ago. You can send individual books to the kindle from manage content and devices but you can't send a collections folder. I don't necessarily download all my collections. What I'm finding interesting and very frustrating is the collections that are missing are the ones I created on my Oasis (my new kindle). However, all of them show up in Manage Content and Devices.
I agree. It is very odd. All my Collections are visible on the Content and Devices page on the website. What is frustrating is that All of the Collections I created on my Oasis kindle prior to reset, will not sync from Content and Devices to my Oasis. I even purposely reset my kindle Voyage. The collections that will not sync on the Voyage are the ones originally created on the Oasis. I thought the problem was with the Oasis but I'm not so sure.
and you don't see them at all on your Oasis after you turn on wireless and sync (menu option or icon) and you turn on the Voyage and sync and then set the Home page on both to look like this:
All (Highlighted) | Downloaded | Collections ___ Title
you should see the same Collection names on both - if you don't try another sync on both. Another possibility is that you have two linked (Family Library) accounts and the two devices are registered to different accounts. Collections are not shared, just the books in them, but you can move Collections to the otehr account (I checked and this is an option on the Content and Devices page)
What the means is that either they were created on a Device that doesn't support Cloud Collections or the Device was not synced to the account after they were created. Keep in mind that if you delete a Cloud Collection on one device, it will be removed from your account and from all the other devices that support Cloud Collections, so if you have another device with Cloud Collections and deleted them from that device, they're gone permanently