I had a few collections on the Amazon Kindle web interface populated with books purchased through Amazon, readable through the web reader. However, I wasn't able get my Kindle to sync with the cloud so that they even showed as available until resetting the Kindle to factory defaults.
I confirmed that books were being downloaded, that there was sufficient free space. And, yet, the Kindle showed the collections as empty until it was reset to factory defaults.
I even selected the items and attempted to deliver them to the specific device, which was reported as being successful. However, the pending deliveries remained unchanged.
It was as if the items weren't showing due to incompatibility with the Kindle reader -- and yet, clearly, they were compatible. How do I know that the items are compatible? Because, by resetting the Kindle to factory defaults the content loads. But, and here's the key point: only after resetting the Kindle.
Therefore, the problem lies with the Kindle itself, or at least how it syncs -- and not with the content. Yet, it seems like the content is incompatible.
That salient point being that other users report that what shows on
Manage Your Content, the web interface, does not get similarly reflected on a Kindle e-book device. What I describe is a little different, but, probably, the result of the same underlying bug.