Sorry to burden you with a potential bug Steve. I’ve already raised this as a question on the forum, but nobody has come forward with any suggestions – see: https://getmusicbee.com/forum/index.php?topic=23367.0
In short, if MB is configured so as to use the 'album covers', 'artists' or a 'custom’ view, plugins are only listed in the context menu once, immediately after start-up. Subsequent right clicking an album or track displays the context menu without any plugins that are intended to be listed on it. I’ve established that this issue is dependent on the view that's being used - it doesn’t occur if the view is 'Tracks' or 'Album and Tracks'.