I have 2 Plex instances running,
One on a TerraMaster NAS, and the other on an Ubuntu server.
Both run off their own media folders, on their own storage.
They are in sync, so both have the same files.
Both have the file “Ford v Ferrari.mp4” in the storage.
One Plex instance (Ubuntu) has listed the file, the other has not 
The TerraMaster is updating with new additions.
Both instances seem to be configured the same.
Any thoughts how to investigate this?
Have you checked if the TerraMaster instance for duplicates?
Could be embedded metadata enabled/preferred on one instance but not the other?!
Many thanks for the quick reply.
Both are disabled:
Just a quick update to say that I rebooted the TerraMaster, this morning, and now the film has been added.
Now to work out why one has 944 films and the other has 946 
Then why they both actually have 957 films on the storage.
Sort both libraries by something other than „By Title“ to get the actual number of movies… if sorted by title the library could hide items which are part of collections.
If that doesn’t identify the missing items, recheck the library for duplicates (-> filter).
Edit:
As a last resort you can use a tool like ExtractTools to pin down the exact differences (e.g. doing a side-by-side comparison of the resulting exports)
