This seemed to happen after one of the updates, but certain shows suddenly are not in my library, even though they are on my hard drive. The only things affected seem to be TV shows, like I love lucy. Any suggestions?
I have tried to name it four or five different ways, including (complete series), season 1-…, Etc. Plex will not find it, when it was there before the update.
Have you tried using the id in the folder name so plex will force match it against what you want. So, for this example and I Love Lucy - TheTVDB.com you can use I Love Lucy (1951) {tvdb-70584} which should then match it.
The additional information such as the year or the online ID definitely helps Plex when there are shows/movies with similar names (I Love Lucy / The Lucy Show), reboots (Magnum, P.I. / Magnum P.I.), multiple movies in a franchise (Star Wars, Harry Potter), etc.
Specifically to TV shows, use a season folder, even if only one season and use sXXeYY in the file name to denote season and episode.
See documentation linked above for additional details.