Solution that might work: I did have exactly the same problems - none of the MP4's created with those converters would drag onto my walkman through windows explorer (incorrect format, skip, skip all, blah blah). Then.... i installed Windows Media Player 11. And now all the mp4's work fine! I can only assume that microsoft bundle new versions of "Media Transfer Protocol" libraries (needed for the dragging from explorer) with Windows Media Player 11? Sounds about incestuous enough for MS. Anyway, install or reinstall Media Player 11 and it might solve your problems - worked for me.
