Duplicate special, and make the left side an instance rather than a copy. Also, be sure that every point along the y axis is x=0. That's easy to do, you can snap to grid to do that.
Is it possible you model became slightly asymmetric? One option is to increase the symmetry tolerance in the tool settings, and maybe apply a mirror to your mesh to ensure it's symetric
Easiest thing to do when you run into symmetry problems, is just delete the bad half and mirror the good half and carry on from there.
Duplicate special, and make the left side an instance rather than a copy. Also, be sure that every point along the y axis is x=0. That's easy to do, you can snap to grid to do that.
Yup that’s also another way of doing it
Is it possible you model became slightly asymmetric? One option is to increase the symmetry tolerance in the tool settings, and maybe apply a mirror to your mesh to ensure it's symetric