Just got a new mouse, and...

…the scroll wheel doesn’t seem to work. If I click the wheel, the little up-and-down thingy appears and works, but things don’t scroll when I just roll the wheel. The troubleshooting whatsit is no help at all. What could be wrong?

(It’s a fairly simple Logitech optical mouse. No bells and whistles.)

EDIT: A friend fixed the problem for me. Lock/delete this as needed.