Yeah I totally have had that happen to me. by moving your center point to one of the verts that are aligned with the center when you hit your mirror geometry you'll get the results you are looking for.

I have had issues with the mirror geometry not always connecting ALL the verts which is weird which is why sometime I just dup the object and then then scale it -1 in either X or whatever transform it is in, in this case I'd say (scale x : -1) but you will have to merge your verts by hand. If all else fails.

Sean