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?