Hey all,
It looks to me as only 2 things will cause this issue,
1, The origin is not in the centre of the object, on the mirror axis, in this case the x axis.
2, The geometry is not symmetrical on the x axis. If you moved the verts, even a fraction then turned on x mirror, it will not work.

To solve this I suggest add an edge loop in the centre, delete one side of the mesh, add a mirror modifier, enable clipping on the modifier, move the edge loop you added in the x direction until they stick in the centre, apply the mirror modifier, delete the edge loop and continue.