I have a well functioning library for Movies, TV-shows and Photos - but a folder with other videos, such as personal video files and a lot of music concerts, will not appear in Plex. According to for instance this support page (Creating Libraries | Plex Support) I am supposed to have the option to BROWSE FOR MEDIA FOLDER but that option is nowhere to be found. All handle or add Library windows lack that option - and the agents do not find anything. What to do?
Do you have an ad blocker in your browser? There have been a few cases where an ad blocker cause a feature to not appear in Plex. If you have an ad blocker, try disabling it temporarily and see if the missing elements show up.
Or are you using a WD MyCloud Home as your Plex Media Server? For those, the folders linked to a library are hard-coded… any movie type library will point to the folder ./Plex/Shared Movies/ etc.