First of all, iTunes has a nasty habit of switching from mapped network drives to the local default if the NAS takes too long to respond, usually when Windows disconnects the network drive because you’re not using it.
I’ve found that it’s more reliable if you tell it the real path to your music.
i.e. \nasnamesharenameMy Music……..
In iTunes select “Advanced (pull down menu)”>”Consolidate library”.
If that doesn’t work for you then you’ll have to “Add ***** to library (in the file pull down menu)”