You can try this: go to Options -> Locations -> VST Plug-Ins and click the button that says "Reset The Blocklist". Then restart S1
If that does not work:
You can bring up the Plugin Manager (on the Mac, it's View -> Plug-in Manager), you can remove all of your plugin settings. That will force S1 to rescan all of the plugins. This might be kind of extreme, but it's the only solution I know of if resetting the blocklist fails.