Unable to find/add to lib: Dexter new blood

You could edit the original Dexter show‘s 1st episode (:pencil2:) to see on the Info tab if the 1st episode of your new show is mismatched to that (same works using the Get Info action from the context menu).

Given Plex had already matched the files, it’ll insist to keep that (wrong) match. There’s two scenarios to approach this:

  1. Follow the Plex Dance procedure to make Plex forget that previous match/association
  2. Click Split Apart from the show-level context menu, then fix-match the Dexter: New Blood files