Have you tried using the 'Folder Search' option, which is one of the Music Search options? You don't say what Media option you have on your car; it may be that this is only part of the Media XL package (which I have)
Also, bear in mind that if you have a large number of tracks on a large USB stick, it does take some time to load...if your stick has a progress LED light on it, you won't see the full content until the light stops flashing. I have also read elsewhere that you can get time lag, and problems, with very large USB sticks (64GB +)
Maybe try using a smaller capacity stick, like 8GB and see whether that helps?