Hi,
We are seeing reports of users installing plugins, opening live and they are not present in the plugin list. By going to preferences and deselecting the option VST/AU custom path, and then switching it on again the plugins will now appear in the plugin list.
If you ever experienced exactly this behaviour, please send in your preferences.cfg to support@ableton.com and note that this file was requested in the 8.4 forum by me. Also list which plugins is affected by this, and the plugin version number.
Best,
Mads