there is something different between your three test files. perhaps size is the issue.
Can you play the files by accesing them with Samba (windows explorer)?
Can you play the files when they are streamed to Windows Media Player?
Can you play all the files with DLNA server streaming?
What version of Twonky are you using? Have you ever attempted to update it?
Lot of things you can think through to rule out possible causes.
