Probably the feature described by the sketch is mirrored. You will need to delete the mirror, make the change and remirror. Usually the mirror command is not as useful as it seems. -- Mark von Huben
thanks for your reply. actually, i found a clue that I`m not sure, all the datums in the reference of this sketch are in a same pattern.
In Reply to Mark Von Huben:
Probably the feature described by the sketch is mirrored. You will need to delete the mirror, make the change and remirror. Usually the mirror command is not as useful as it seems. -- Mark von Huben