When the provisional symmetry axes are in the desired position, they must be applied (finalized) for the artwork to actually be mirrored. There are several ways to do this:
The Apply to Selection/Layer button on the panel may be clicked.
The Enter
key may be pressed. If Selected Artwork mode is active, the axes will remain in place (but become provisional again) after the selected artwork has been mirrored.
If the tool preference Display Annotation Dialog is enabled (as it is by default), a small on-screen dialog will be displayed, offering three clickable menu items: Apply to Selection (unavailable if nothing is selected), Apply to Layer (persistent), and Cancel, which releases the provisional axes so they begin following the cursor again.
MirrorMe Annotation Dialog