Are you trying to flip your mesh to the mirror image or re-introduce symmetry?
If you are trying to reintroduce symmetry, try the following:
Make sure only an object that should be symmetrical is visible. Hit Transform -> S. Pivot. This will put the pivot in the geographic middle of what is visible.
Now, mask the half of the heads you like, leaving the “broken half” unmasked.
Go to Tools -> Deformation -> Smart Resym. Make sure that only the little “x” on the right side of the button is highlighted. This will attempt to re-adjust the symmetry of the unmasked part, using the masked part as the template.
HTH,
-K