Not all files types are supported. Below is a page from the manual that should list out compatible formats that will will with DLNA:
The video codec, audio codec and resolution must match up for the file to work properly.
I have a new 40" Samsung LED TV and have it hooked up to my PC via an ethernet connection. However, whilst I have been successful in sharing photos from my pc using samsung's file sharing software, I can't seem to access the movies i have stored in itunes. I think it may be something to do with .m4v files, and I've converted a couple of movies to mpegs - but still no success. All I get is a blank screen... Any ideas!?