Which two series is it?
Some series are represented differently in TMDB than TVDB. Plex uses the numbering system of TMDB by default, so if you are expecting it to be the series from TVDB you might have issues.
Do the Plex Dance on the files, and if it still doesn’t show up, use the tip Ozsmac posted about forcing Plex to match the folder automatically to the right show using the {tvdb-TVDB_ID_HERE} text at the end of the folder name.
Another thing to check is that you are properly targeting the correct folder for the library itself.
