The knife cut won't be symmetrized automatically with the X symmetry option on, this tool will only move or remove the vertices that are moved or removed on the opposite side. What you can do though is select the new edge and Mesh > Symmetrize and it will be mirrored:
Also, the symmetry will work as long as you keep the other side symmetrical, if you begin to change it unsymmetrically, you are breaking the symmetrey and you won't be able to do any symmetrical operation: