Shape data not displaying in callout

[Planethead]

I have created a custom callout to display shape data, in this case, the identifier of a shape in a schematic.  It works great when I first drop the callout on the sheet, connect it to an item, and select the data to display from the dialogue box.

Later, though, some of the callouts seem to lose their link to the shape, and if I delete the callout and try adding a new one, the "Shape data" portion of the Configure Callout dialogue box is empty, even though the shape still shows that it has data attached to it.  By adding a new copy of the shape, I can usually link to it.

In the process of writing this question, I found the problem, I think.  I had placed a rectangle around the entire schematic after placing most of the shapes as a visual frame.  Even though it was unfilled, the rectangle was still "above" some of the shapes, so trying to link a callout to a shape just linked it to the rectangle.  At least that's the theory, because once I sent the rectangle to the back, the problem went away.

Thought I would share, even though my problem seems to be solved, in case anyone else had the same issue.