Before re-scanning your installed plug-ins, make sure the search path includes the directory where your Drones have been put when you installed Synfire (default: C:\Program Files\Steinberg\VstPlugins). The search path is shown in a dialog window that pops up immediately after you requested a scan. On Windows, different directories apply for 32 and 64 bit plug-ins.
In case you accidentally messed with the 32 and 64 bit plugin files (Windows only), you can uninstall and reinstall the Audio Engine and point the installer to the directory where your current VSTs are installed. This will place Cognitone plugins along with your other VST (always uninstall using the Windows's control panel).
If Drones in your DAW do not show up in Synfire, the version of your installed Audio Engine may be outdated or not matching the version required by Synfire. This could possibly be because you have selected a different VST directory on a recent reinstall, but your DAW still uses the old ones. Please reinstall Synfire including the Audio Engine (always make a backup of your projects first. On Windows, uninstall using the Windows's control panel only. On the Mac, your software download includes an uninstaller app).
A firewall or security utility may be blocking Drones from connecting with Synfire via TCP/IP. Check if loopback connections to 127.0.0.1 (localhost), or any connections to port 8082 are barred. Although this is very unusual, unless you are running Audio Engines on different computers in your network.