As of the last couple of server updates Extras do not display in any items in the library type of Other Videos (they did display near the end of last year). This is apparent on Windows PC server, Windows Plex Web and Windows Plex Player. Oddly, the Windows Plex Media Player works fine as do players for iOS. There is no problem with Extras for libraries of the Movies type, just the Other Videos type.
I chose to have an Other Videos library for personal stuff. A good example as to why Extras would be nice to work is that I have a full concert of a local band with extras that are shorter promo videos and single song videos. Placing these types in a Movies type library doesn’t make a lot of sense.
Seems to be some hiccup with Plex Web not loading the extras hub.
I can reproduce this in my home videos library (Other Videos w/ scanner/agent set as specified by Otto).
The extras don’t show in Plex Web but they still show my phone (iOS).
Yes, I have the scanner and agent settings as Otto says above. I researched all this prior to my first post to make sure I wasn’t screwing up on my end. As I stated earlier, Extras showing up worked fine until about 2 or 3 server upgrades ago.