As /u/rezz@lemmy.world said, you can try to force a scan of the library. Log into the admin and hit the big "Scan All Libraries" button, then give it some time.

A refresh of that page should show a progress meter.

In order to encourage more accurate detection (assuming it can find/access the new file at all), there are advised naming schemes for your files. See here for a basic overview: https://jellyfin.org/docs/general/server/media/movies

I prefer to include the full name, year and imdb info of a movie, ie Citizen Kane isn't just "Citizen.Kane.mp4", it's:
"Citizen Kane (1941) [imdbid-tt0033467].mp4"
based on the information that's publically available here https://www.imdb.com/title/tt0033467/

Even if you don't enable imdb itself for the metadata lookup, that will give you an almost guaranteed detection during a library scan.

If this sounds like too much work, there's several automated tools for naming your personal dvd rips, such as Radarr.

If it's still not being detected, it's time to dig into the logs and find out WHY it's erroring.
Is it permissions?
Is it naming?
Is it the phase of the moon?

You can configure Jellyfin to automatically add new found items to your library but you will need to get on top your logs to make the most of this:

• If you are getting scanning errors then you need to look into them
• (Building on above) You will find that your OS has limits on how many file watchers it can have. You may need to update your config to accommodate changes.

Did you go to Settings in your desktop server instance of the app and Scan Library? This updates it manually everytime you add new files.

Also make sure your directories are appropriately structure with Movies and Shows based on their advised format of folder nesting and naming.

Try scanning the libraries again as other users already pointed out, this works for me every time I add a new media.

Just throwing it in there, if you have your server on Linux a ton of times having the right permissions for files is key yo get them recognised. If Jellyfin cannot read the files it won't find them. I assume that's no issue for you since you already have a library, but still wanted to mention it.