[mising]

Am I missing something ridiculously simple here?
How do I get side loaded content on my KFHD to show up and sync on my Oasis and vice versa?
On my phone and old tablet with the Kindle app I can see everything, but on the KFHD, the only things that sync are the books that I have purchased via Amazon.
Is there a particular folder I can put them in?
If I put them in the /emulated/0/Documents folder, they show up in the KFHD Books Library but still do not sync with the Oasis.