You can filter your library for Unmatched items. That should make it easier to catch up on those not working properly.
Generally, Plex should be able to match the vast majority of movies you throw at it… if you stick to the naming schema and/or provide it with hints where necessary (e.g. Movie Title (year) or Movie Title (year) {imdb-tt1234567}). As you just found, you can also manually fix items that didn’t properly match right away.
