Recently, I bought a PS3, and connected it to WD MyBookWorld. I was adding stuff to the external disk, and sometimes it did not show on PS3.
It was all down to the media server on WD MyBookWorld end.
Here are few things to check, if you are having similar issues:

1) Browse to the web interface of media server running on external nas drive (MyBookWorld in my case).
2) Make sure there are no ‘foreign content’ i.e. something other than Music, Videos, Pictures, etc. If you have a bit folder with loads of small files, it will make the process of indexing on NAS side very slow.
3) Normally, there is a way to tell the media server to index a specific folder for specific content. This should be quickest. i.e. make three folders – Music, Movies, Pictures, and index accordingly.
4) Make sure the indexing has happened. (from logs.)