A quite common topic we get in support is that a sheet metal part has been created and the flat pattern generated no problem. Then a new part has been derived from it (an opposite hand version for instance) and when opened and the flat pattern is generated it fails. To all intents and purposes they are the same part, so why does the new one not work?
Generally the answer is to do with material thickness. The original part may have had its Sheet Metal thickness set in the Sheet Metal defaults option. The new part will be using the default Sheet Metal rule thickness. If these are different, the derived part will not be able to be flattened. Changing the sheet metal thicknss to match the orignal part solves the issue
Have a look at this short video to better explain the above
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