Have you tried resetting the plugin cache? It may well be that there is an issue there.
I don’t know where it lives on a Mac, but the files you need to delete (and rebuild) on a PC are called “VstPlugInfo.xml” and “VstPlugInfoV2.xml” and they are both in the prefs folder.
There will probably be similar files for AU versions, and if you delete these it will force Nuendo to rescan all VST2 plugins and rebuild it’s internal database of them and the odds are this will fix your problem - give it a try!
(NB - slight downside - it will lose any custom VST2 plugin folders you have defined, and these will need to be put back in the plugin information screen - do not do a rescan from there either - close & restart Nuendo instead)