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Hello @APersistant-Beginner
Did we not cover this in your post on the same subject the other day?
In order for mirror symmetry to work on a mesh, the mesh itself must be symmetrical along that axis. Symmetrical means “the same (but reversed) on both sides of the axis plane”.
Your mesh is symmetrical along the X-axis, which is why mirror symmetry works there. It is not symmetrical along either the Y or Z axis, hence the reason it fails.
There is, generally speaking, no opportunity for symmetrical sculpting (or masking) along the y or z axes of most humanoid figures.