3 boxes, one 'almost invisible'

I have three HDR-FOX T2s on my homeplug network and all are running the latest customised firmware. All three have been installed for some weeks now.

I can access all three using Firefox and web-if (192.168.1.???) and I can see all three in Windows Media Player.

However, if I try to find them on the network using Windows 7, only two are visible and, typically, it's the one that's missing that I need to transfer some recordings to.

As far as I can see, the installations are identical other than 'webif-epgkeywords' being present in the list of installed packages on the 'missing' box.

Anyone any idea on what the problem might be?
Yes, and checking just now has proved the power of a forum post. Mysteriously, after weeks of waiting, the third box has announced its presence on the network...