If you have a Failed to Mirror (x-amount) of Vertices, there are likely mesh errors (doubles, flipped normals, etc.) or your mesh is not symmetrical. I can see from your screens that it’s the latter because of the hair.
You could easily remove the hair and treat it as a separate object to regain symmetry in the face, but you will be destroying your Basis shape and any other shapekeys you have made.
Try to create a workflow that will allow you to never make changes to the mesh after you have invested time in creating shapekeys.
The basis shape is the road map for all other shapekeys. If you change the vertex count in your model, (like you did when adding the belly button) you will likely break all of your other shapes because you are changing the number and position of those vertices in the basis.
More often then not, you will be forced to delete all of the shapes and start over with a new basis.
Luckily you are learning this early and can avoid this problem in the future.