I don't know the answer but there are some things you might test to see if you can work out why the union works some times and not at other times. When it goes bad, undo and then do some of the following tests...

Does the order of selection before applying the merge make a difference?

Are both shapes the same type? If you copy each of them and then go to Paste Special, what is the default Paste format?

If both are 'Block Arc' shapes with the yellow handles, has either been rotated or reflected before the merge?

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Andrew Lockton
Chrysalis Design, Melbourne Australia