After removing all components and plugins and rebooting, I got the amazon links appearing as correct oneboxes.
I then installed discourse-adplugin and immediately went back to 9 failures out of 10 tries. When I turned it off, I got a couple to work, and then it returned to the failure mode of not obtaining the description.
I removed the discourse-adplugin from app.yml and rebuilt it. Then, I rebooted the server. Amazon links started working again with onebox.
There must be some conflict with the combination of plugins and components that I am using. What options are there to debug this?