Are the sticks you're trying of different brands/capacities or are they the same product? I had the same problem with an SD card, but it was only 1 card. When I tried a different card, it worked fine. The two were very different in size (2GB vs. 8 or 16GB) and I think someone has started a thread where people can list which cards do/don't seem to be recognized.
Maybe we need a similar thread for USB flash drives.