Block diagram expansion criteria

[m3nth]

Sometimes when I hold down Ctrl and drag a box or line on the diagram
in order to expand it, the diagram does not expand.  In the areas
where this happens, sometimes I can do the same thing repeatedly with
no results--there is no expansion of the code.  Does anyone know why
this happens or what the criteria are for it to work?  Where I'm
drawing the box is usually in the midst of code, ie, it is not a
'free' area where there is lots of space, such as within a wide open
do-loop.