You didn't give us information about the kind of hardware your using, so I can only take a guess. There have been reports that the webcam you're using will only work in usb 2.0 ports. First you can try plugging it into other usb ports on the computer and see if it shows up in lsusb.

If it shows up like this, it may be plugged into a usb 1.1 port:
Bus 001 Device 002: ID 1e4e:0100

If it shows up like this, it may be in a usb 2.0 port:
Bus 001 Device 002: ID 1e4e:0100 USB 2.0 Camera Etron Technologies [10]

If you still have a problem, show us the printout of lsusb (with the webcam plugged in).