USB Flash Drive Not Recognized

I have a school computer lab that we recently re-imaged, which we do every year, with a clean image. All the computers are Dell Dimension 2350's. Since re-imaging only select usb flash drives are "seen". Some flash drives are not seen in my computer but do show up in device manager as needing drivers. Once the drivers are loaded they can be accessed through the application, but still not seen in my computer. They do not duplicate drive letters in disk manager. Other flash drives are completely invisible, not in my computer, application or disk manager, nor will the drivers load. Some flash drives work perfectly fine. I should also note that bios. chipset and updates are all current. Device manager does show the PCI Simple Communications Controller missing. Any suggestions?