Router doesn't let devices see each other?

Something is wrong with my router. Connection to the internet works fine, but the router doesn't let other devices see each other. Examples:

Playing multiplayer games (i.e. Minecraft PE) on Android mobile devices doesn't work on this connection, but works on others.
When using AutoRemote and EventGhost, I can't send messages from my phone to my PC.
Chromecast is having trouble connecting to the Wi-Fi.
Computers often disappear from the wireless home network.
IP Address conflict?

It's a Belkin D50, any suggestions? (Other than the usual turn it off and back on please?)

GJB3

Some Belkin routers include a guest network facility. The guest network will allow guests to access the Internet, but they will not be allowed to access other guests' devices. Make sure your devices are connected to your full private network and not to the guest network.

Some routers have a facility to disable communication between devices connected to the full private network. There should be a setting to enable or disable this feature. Check the routers configuration pages to see if your router has this facility, and if it has, how it is currently set.

Richard