I'm trying to make body presets with Cmar's Morphmaker, but I'm having trouble with the actual mesh editing part in Blender. When I try to move the vertices on the mesh, this happens:



As you can see, some of the vertices on the mirrored side don't move, and I don't know why this is. I've heard that it could be because the mesh is slightly asymmetrical, but in other video tutorials, the mesh seems to move mirrored completely fine. I know that in Morphmaker, only the sim's left side is selected and the other side is mirrored, but I'd just like to know if/what I'm doing wrong.

I'm using Blender 4.1, so I don't know if some sort of change has happened between it and other versions.