I have tried rebooting the computer & Apple TV and deleting cache file which automatically get generated in the photograph folder. I have experienced the problem on two networks using similar devices. Firmware and iTunes softwware are up to date.
This seems such a simple thing to do but seems to be unreliable.
Last time I had to resort to using Photo Stream rather than a networked computer. However, at work I want network area where staff can place photographs to be shared across several screens.