The thing to keep in mind about the PS3 and flash drives, is that if you don't put the media in a specifically named directory the PS3 will sometimes have issues finding them.
VIDEO
MUSIC
All caps, and just those names (for music files and video files obviously) I don't use it much but I believe pictures are PHOTO. If you ever run into issues again give that a shot.
The other thing you can do, is if you have a computer you leave on, or one you don't mind turning on when you want to stream something to the PS3, check out "PS3 Media Server"
http://ps3mediaserver.blogspot.com/
I've been using it for a while to stream from my file server, and it's been working like a dream. :)