Just to be sure we're not dealing with caching and so forth, if you double-click on the networked photo in Nautilus, does it appear in EoG (or whatever viewer you have configured)? After doing that, restart Shotwell. Does it still appear as a Missing File?
I know this seems redundant, but it helps to remove certain ambiguities so we can reproduce it here.
Just to be sure we're not dealing with caching and so forth, if you double-click on the networked photo in Nautilus, does it appear in EoG (or whatever viewer you have configured)? After doing that, restart Shotwell. Does it still appear as a Missing File?
I know this seems redundant, but it helps to remove certain ambiguities so we can reproduce it here.