I think that iTunes won’t spin up a network drive if it’s mapped to a windows drive letter. Not sure if Windoze is not recognising the access request or iTunes is timing out.
I do know that Windows likes to disconnect mapped network drive if there is no activity on that drive.
When you use the actual path as the library location (i.e. \servernamesharename) iTunes starts to behave itself.
NB: I’ve seen iTunes reset the library location to the default because it can’t connect to the mapped drive. Rather annoying behaviour…