|USB devices may not work correctly when they're connected to certain USB ports e.g. ports in a hub or on the front of a desktop case. If you have a desktop computer, you may have an IRQ sharing problem you need to fix, or your power supply may be starting to fail. |
See response 3 in this:
Also - if you are using a USB extension cable, some of them have inadequate wiring and will not work properly with some USB devices.
The scanner must be powered and be on. Some have a power switch, some don't but must be set to a mode so they're ready for communication with the computer.
Many scanners have a locking device meant for preventing physical damage when you move it from place to place. If that locking device is engaged, the computer may not detect the scanner as ready for communication with the computer.