WD Live TV Plus HD Media Player wont play .mkv over network

I have a  (NUS2000- Cirago) running as media server and my pc will play all the mkv but my WD live TV plus HD media player wont play .mkv files. When I connect the HDD to the WD device it plays. But over the network it wont play. The cirago and WD are connected to the router with a physical cable. Any ideas?

Initially when my pc was running as a media server, it wouldnt play as well but I solved the problem by installing some divx stuff from some other website and it played fine. But I cant install anything on cirago and I dont want my pc to be on all the time.

Any help is much appreciated.

Check if you have the latest firmware version.

Yes it has the latest firmware update. Any other ideas?