Plex won't see S01e01 but renaming it, it sees it

Hi,
I’m running the latest media server on my Synology and everything has been working fine but.
I recently added the new series of Chernobyl to my server and when scanning library files it picks up episodes 2-5 but not the 1st episode even thou its in the same directory.
I deleted the first episode from my drive and re added it but still doesn’t see it.
If I rename it from s01e01 to S01e06 and run the scan library files it sees it and adds it.
But Just won’t add it when named s01e01.

Any ideas?

I suppose you are adding it to a ‘tv show’-type library, right?
Perform the Plex Dance with the file (or the full series).

thanks OttoKerner, not heard of the Plex dance but following the instructions it seems to have worked. Thankyou