Perhaps it's a compatibility issue. On the back of the packaging, it will say stuff like "Windows XP, Windows Vista, Windows Vista Compatible"
Another thing is (which I personally experienced myself) is that I had somehow damaged my USB port when I tried inserting a flash drive into my one month old laptop. The "four-connection prongs" were bent and twisted. Perhaps it was my clumsiness or perhaps it was the "so-so" build quality of my laptop's components (or both), but the flash drive would not get recognized by the laptop. After I got the USB ports replaced, then the laptop once again recognized my flash drive.