Codad - it could be any number of different things....
First - UpdateMedia is a process that runs regularly and scans any media folders that have been added for new/updated/deleted media. If you only have a small amount of media this will happen within a couple of minutes and populate your database. At that point you should be able to see all your media. If you have a lot of media - particularly if there is a very large folder - it will take longer, as it only writes the things it finds into the database after scanning a folder.
If you go to the web admin, Media File Sync menu, you can navigate to the folder and force a sync in the foreground - that way you can see when it completes and updates the database. Check at the end of the log window and make sure it isn't saying that your device is offline. Alternatively, look in the properties of the share at the bottom of the page - is the Online tick box ticked?
If not, and the log window says the device is offline then that is the issue - even if your database is populated, if it thinks the device is offline it will not display your media. I have had issues where only a random sample of media appears shortly after adding a share, and each time I go in it is a different set (or single file) from many, and eventually they disappear altogether. In that instance, the database was populated, but it thought the device was offline so didn't display the media. This was a file share protocol issue. but check the other stuff first, and maybe look in Media File Sync as well to determine if the media is actually in the database or not (should be a big tick graphic next to each item)
