Oval autoshapes revert to rectangular boxes

I have an Excel spreadsheet on which I have created 'oval' Autoshapes using
Addshape (msoShapeOval). These 'ovals' contain text and have various
coloured backgrounds. When I re-open the spreadsheet, some (but not all) of
the 'ovals' have converted themselves to 'rectangles'. The text they
contain, and the background colour are unaffected.

I would be grateful for any explanation and/or advice colleagues could offer.
--
Nigel