You've probably sorted it or given up by now but let me give this possible solution for others who like me had this issue.
In my case, I thought it might be that I was using the AU version of the plugin, and sure enough, when I replaced the plugin with the VST3 version the fader control worked. The AU versions just don't work. So if you are on a Mac, try that first.
Best not to use AU versions, I finally figured out.
Best of Luck.