I think it has to do with some timing problem. I'm not a network guru so I couldn't even begin to tell you what it is.
However although not a fix I do have a work around that I use.
Try typing in...

As the title says I am unable to "see" my NAS in windows explorer. I was able to use explorer to view/add/delete files on my NAS a few weeks ago but now now cannot even see the NAS in the network...