Listbox scroll bar won't scroll!

I have an application with a listbox that won't scroll. The scroll bar looks
perfectly normal -- proportioned correctly (vis a vis number of items
showing/not showing in listbox). It just doesn't work.

It's a normal, vertical scrollbar -- I haven't added a horizontal bar or
anything elaborate like that.

This listbox has style lbOwnerDrawFixed, but my OnDrawItem event for it
(which works fine) is not kinky.

The app. has a couple of other listboxes and they're all working fine.

Any leads on solving this much appreciated!

--
David Howorth