I've had this problem for a while. I have a couple of computers connected on my home wireless network which generally can see each other to share files. But sometimes I'm unable to connect to the other machine. I'd always put this down to some weirdness going on in my wireless network and dealt with it by copying files onto a USB drive. But I finally figured it out today. It would appear if I have the SonicWall VPN client running on one of the machines this causes the wireless network to go funny. Shutting SonicWall down fixes the issue. No idea if this is a bug or if it is required behaviour but now I know why it happens I don't really care.