Aperture Scroll Bars Don't Work

I seem to be missing a switch, option, Aperture isn't working as Cupertino intended or I failed to RTFM...

In 'Browser & Viewer' view with the Browser in 'Filmstrip' (both set through the View menu or by the View drop down top right in the default toolbar) neither the scroll bar or fast forward / rewind nudge work. Change the Browser to Grid and they do work. Or, put another way, scrolling horizontally is frozen but vertically is fine.

In both cases, changing one photo at a time in the Viewer window using cursor keys is aOK.

Can anyone point me how to get scrolling in film strip working?

--
Andrew