Missing movie titles via both media server or media share

I have about 150 movies in my movie collection stored in NAS.  Just finish all the ThumbGen process using Black Momba Theme.  I see all the movies except 2 titles… tried changing movie name, redo thumbnails, recreate movie sheet, unit reset, power down, reset everything including NAS & router, delete all .wd_tv folders… still no luck.  I can only stream the two titles through browse file method.  It’s not a deal breaker by any means, but it would be great if someone knows whats causing the issue.  I have all 4 files for both titles in the folder, the .jpg .mkv_sheet .xml plus the video file .rmvb in both cases.  Is it the file type issue??  Any suggestion would be greatly appreciated.

Maybe the NAS media server might not be compatible with the .mkv file you are trying to play. 

RMVB isn’t supported.