Mendeley picks up information from the PDFs and websites you import from. This is not an exact science, so it is ALWAYS a good idea to review and edit your references accordingly. Any references that need to be reviewed will have this icon:
In Mendeley Desktop:
After adding references either from PDFs or the web, you can edit the references if information is missing or incorrect. This video explains how to search by DOI, by title in Google Scholar, and manually edit information.