Try these solutions:
1. Ensure your chosen UPnP server software can serve the file type you are trying to access
Some servers do not serve FLAC for instance.
2. Make sure the files are non-DRM
The Network Player can only play non-DRM files. Files that have DRM controls cannot be played by the Network Player. Some servers may be able to authorise and de-crypt Microsoft PlaysForSure before then serving the content to the Network Player but this operation is not guaranteed as it is purely a function of the server.
3. If attempting to play 24-bit WAV or FLAC content note a wired Ethernet connection is normally required for reliable operation due to bandwidth considerations
4. Check that the Network Player can play back the file type you are trying to access
WMA, AAC, MP3, Vorbis, FLAC and WAV can currently be played. The Network Player does not support ALAC, WMA Pro, WMA lossless, RAW, AU or other file types not mentioned.