In that case it generally is the Server that would be missing the files. Another client (app/hardware) could help to confirm this.

I simply select upnp server on MM, it finds the NAS drive, I navigate to the MP3 folder and play.

I'll look to see if the Netgear NAS 104 has auto scanning

Thanks

If you're using the UPnp(DLNA) server on the NAS it's up to the NAS to scan for the new files (should be automatic if you put the files in the right place). Then under UPnP you should see the files as well.

How do you browse NAS from Windows Explorer and what is our main music path?

My MM quite happily browses and plays MP3files from my NAS drive, using the upnp server etc.
But I have added lots of new folders to the MP3 folder, but MM does not see the newer files.

How do I resolve this?

Thanks