Studio One Blocklist Reset Guide
To make Studio One rescan a plugin that previous had problems, you'll have to reset it's plug-in blocklist and restart the DAW so that it scans through those plugins again. You can find this in Preferences > Locations > VST Plug-Ins as shown below.
Note for Windows users: The directory in the "Locations" list will be different. Unless you have a custom VST location, it should contain "C:\Program Files\Vstplugins".
Click the Reset Blocklist button, and confirm the pop up message. Make sure Scan at startup is enabled, and then restart Studio One. The plug-in should be scanned and if it's not activated yet, the activation pop-up window will appear. You can check out this guide if you need help with the pop-up window activation process.
In some cases, we have noticed that the above solution doesn't end up making Studio One rescan Soundtoys plug-ins. The alternative solution to this is to delete Studio One's plugin cache so that it is forced to rescan all the plug-ins you have installed again.