If I simplify the way you give the id, it becomes this:
var c = a.id = circleName;
Because of the second =
, c
is not a circle
element, and the tooltip cannot be shown, it does not know where.
If you set the id after creating the circle...
var c = self.paper.circle(x, y, radius)
.attr({'fill':'#f2f2f2'});
c.id = circleName;
$(c.node).qtip({content:{text:circleName}});
... the problem disappears.
I created a JS Bin to show you (not exactly the same code but it should be enough).