Start by disabling your firewalls / A/V software temporarily to see if that is blocking the DLNA traffic.
Also check that your renderers are on the same IP subnet *) as your server(s) and control point(s). If parties are on different subnets, then it is possible that UPnP Alive notifications might not be routed across all parties. (It depends somewhat on your router.)
*) note being on the same subnet means (non technical answer) that all parties have an IP Address starting with the same major digits.
You can either manually set up your server(s), control point(s) and renderer(s) to have an IP Address on the same sub-net. Or alternatively (much easier) if your router has a DHCP server function, then enable that function, and set up all your server(s), control point(s) and renderer(s) to use DHCP to get their IP Address.
PS note that it is possible to have a surfeit of good things: if you have more than one router box in your LAN (e.g. an Ethernet cable or DSL modem plus also a wifi router) then both of those boxes could have a DHCP server function in them, and if both are enabled, then you will have a real rat's nest on your hands...