This seems to be an area that few Excel users are familiar with: The Drawing
Tools.

I have been using Excel do create some drawings and found the Excel drawing
tools quite powerful. One feature I use is to enable the Selection Pane,
and then move individual shapes more to the front or to the back, to allow
one shape to block another, or vice versa. After using this for a while,
the Reorder buttons have stopped working. What could cause that? How can I
get them back? Is there a limit to how many individual shapes are allowed
in one worksheet or workbook?

The Bring To Front and Send To Back buttons on the Format (Drawing Tools)
ribbon are still working, even the Bring Forward and Send Backward options,
but the Reorder buttons on the Selection Pane are totally unresponsive. So I
haven't completely lost the functionality, just the more convenient way of
invoking it.

Excel 2007, XP with SR3 and all updates.

Thanks for any help -- Renny