I'm having trouble discovering UPnP/DLNA devices on my network. Discovery seems to be unreliable; I've tried several UPnP device listing applications both on Windows PCs and Android devices. Often, devices are not discovered until I look up their IP address via my router's web interface and send them a ping packet, after which I seem to be able to discover them reliably. I'm connecting wirelessly to a Thomson TG585 v8 router; the device I'm currently trying to discover is a Samsung TV that is connected to the router via a wired connection. I've also previously had trouble on the same network with mDNS services (see this unanswered StackOverflow question I asked while I was trying to get that to work) which may or may not be related.
I've tried discovery with Cling Workbench under Windows, and with several different DLNA media server / control point applications under Android.
Any suggestions what may be causing this, or how best to improve the reliability?