When you click on the Download Article button, ReadCube opens the first link-out provider of the article listed on PubMed or Google Scholar.
However, sometimes your institution does not subscribe to that provider. Or, there is a problem with the webpage you are trying to download the article through. In these cases, go to the drop down menu below the address bar of the browser. Then select a different link-out provider from the list. You will find the the download PDF button on these pages, and can download the PDF directly into your ReadCube library.