This has been a problem with all my Mac computers. To solve it, go into the Web Shield Settings and add "safari-extensions.apple.com" as an https service to Excluded Servers. If that doesn't work, uncheck "Scan secured connections", and in Safari, go back to the "Safari Extensions" website, by clicking it at the top of the Safari menu. If all is well, the upper right corner of the webpage will not display the link promoting El Capitan, and all extensions will show an "Install now" link. When extensions are working as described, you may reenable the "Scan secured connections" setting, including the subcategory "...from browsers only".