M4P's visible to Server, Windows but not plex

I hope I am on the last lap of the setting up of the media server.
Having go all the folders visible, and even now able to see both the PC server and the Ubuntu, the same files types/names are visible to the PC but invisible on the Server. I can see the files in the file manager on Ubuntu.
I have tried the permissions, but no joy.
Now physically exhausted. Need some guidance.