this was driving me crazy!!! here's the solution that worked for me:
If there is a (possibly empty) file named ".nomedia" (without quotations, and make sure you put the dot as the first character) in a folder of your card, then this folder WILL NOT BE SCANNED (if there is a .nomedia in the root of the SD card, then the SD card will NOT be scanned).
So make sure you delete any unwanted of those and then try
"Rescan Media" by Aditya Talpade (android market)