That's usually the sign of a Lua error. In many of the official Lua functions, interactive bits (scroll bars, left- and right-click functionality) are often the last things to load, so they're a pretty good indicator of an error. FireTuner will usually tell you what was wrong.
Assuming, of course, that the scrollbar you're referring to is part of the vanilla game in the first place. If you can't get it to scroll without your mod, then that's a different story.