For the past few years, I've not been about to access the magic items from the Eberron setting that I purchased from the marketplace. I looked into it several times and there are others with this issue. It's to the point that people just chalked it up as a loss, myself included. I can't continue to do that anymore, now that I can't access the monsters too. I want what I've paid for. Does anyone know how to solve this problem??

First, double check that your Account Entitlements are in sync: (https://www.dndbeyond.com/account/marketplace). If they are not in sync, then resync them and check again. If they are already in sync, or the monsters are still not available after syncing, then post again.

They are in sync. They always have been, as far as I can tell. What's worse? More content is now gone... Am I being punished?? My Theros content is now gone, as well...

 It looks like, all I have left is stuff I purchased more recently 😔

I don't understand, at all