I have purchased Xanathar's Guide to Everything on the website. 
On the App it doesn't give me and download option, still labels it with market place and, when I click on it, simply tells me that "User already has a license for this product Via the website"
When I set the App to show "Installed Products only"  Xanathar's Guide is still visible but still has a Market PLace Tag on it and won't give me a download option.
It behaves this way on 2 different Android devices.
I have 13 other books through D&D beyond and all downloaded fine after occasional teething problems which were solved by logging out and back into the App of uninstalling and re-installing.

Somewhere in th "Account" menu of the aop, theres an option to update/download the manifest.  I dont have a mobile device handy to give you more specific directions, but try hunting for that option.  

Thanks for the reply.
I read that in a different thread earlier and unfortunately, it didn't help.

Try this. Click here:

https://www.dndbeyond.com/account/marketplace

and see if it offers you the opportunity to resync that book. If it does, then do that and try it again. If it does not offer you the opportunity to resync then I got nuthin’.

I had the same problem with Tashas’s book because I already had the Oath of Glory but now I can’t use it in the char builder. Is there a way to edit this?

Quote from Makabro >>

I had the same problem with Tashas’s book because I already had the Oath of Glory but now I can’t use it in the char builder. Is there a way to edit this?

 Turn on Magic The Gathering content. Oath of glory was originally published in the Theros book, which is a Magic The Gathering world. 

Thank you!

It worked. =)