If you think one or more channels are supposed to be listed in the Channel Directory but you cannot see them, it could be due to your region settings. Plex channels that are not compatible with your region are only listed under the Foreign (Region-Restricted) Plug-ins section of the Channel Directory.
Channels stream their content from various websites and some of those websites are "geographically restrict" access to the video content. That means users must access the website from within certain countries in order to be allowed to view the videos. In such region-restricted cases, even though you can open and navigate the channel, your IP address must be from the allowed region or regions or the website will deny the request and none of the videos for that channel will play.
The channels that show up in the Channel Directory are ones that are available in your region. If you think one or more channels are supposed to be listed, but are only shown under the "Foreign (Region-Restricted) Plug-ins" section, check your Region settings to make sure it is set properly.
Note: The region setting in Plex does not act like a proxy or VPN. While setting the region in Plex to one other than your own will allow you to install channels restricted to that region, it will not make the channel play videos that are region-restricted.
Related Page: Channels > Region Settings