Our default recommendations engine does rely, in part, on PubMed which is sporadically unavailable. When this happens, recommendations cannot be accessed. However, we have been working on our next generation recommendation algorithm to better find articles based on your library.
To try this out, please head into Preferences and Connectors. At the bottom of that page, you can select the "next generation" beta.
Please let us know if you have any feedback once you start using it by sending us a message here