I gave it a try. The core problem is that the +x side isn't exactly mirror to the -x side.
There are two solutions...
-In model mode select all the CPs on the +x side, on the "untitled" Group that creates in the PWS, choose "Snap to mirrored points"
or
-in the MirrorBone Wizard, raise the "use tolerance for cp position" parameter. I had to raise it to 0.5 to get it to catch everything.
MirrorBone creates a Group, "CPs without mirror" of points it couldn't find mates for to let you know if there are problem CPs.