I assume you have tried the following simple trouble shooting methods, but I'm going to list some anyways just in case.
- Unplug and replug the devices
- Try every usb port on your system and note which ones work and don't
- Try any and every other usb device you have on all usb ports.
- Remove old drivers, install new drivers
The goal, try to eliminate variables. Sometimes you have to multiply x on the top and the bottom so they will cancel out. =)