[Inkscape-user] Bezier curves and making smooth points

Hi,

When I draw bezier curves, there are boxes (smooth points) and diamonds (angles).

Sometimes I can't make a diamond into a smooth point (Shift+S) and othertimes I can. I can't find a pattern to it, Shift+S just refused to smooth some points. Am I doing something wrong?

Cheers, Geoff Russell.