AFCAD apron vertex fixed

I am drawing a big curving apron. I don't know how to make a hollow one just like donut, so I make it like "C" and move the both edge very cloesly, then it looks like "O". Depend on this - keeping "Insert new point", I draw this big apron with one long circle, but something strange happen, in some place I can't activate the green diamond vertex into yellow, say I can't move them ever. Would you please give me some advice? Billy