I had the same troubles in my network. My steps in troubleshooting:
1) Add fixed IP number in television (make sure it fits in the IP and subnet range supported by your router)
2) Add a Switch i.s.o. Modem/router. Original router (Thompson TG712) does not switch Multicast network traffic properly.
3) Try to 'ping' the fixed IP adress you added under 1) and see if the television responds.
4) Turn on debug logging in your Mediaserver and check if something strange happens. i use Serviio, which is freeware and this program has quite extensive debug logging.