I would agree with Neuro to return it, but first a short discussion about this lens from a person that owns it.

My experience has been the wider the lens the tougher it is to get accurate results.

The 24mm is hard to nail when doing an AFMA. For one thing you have a wide DOF with this lens so a AFMA is far less critical than it is with a longer lens.
Because of the wide DOF it will be prone to random results. With the wide lenses you have to be extra critical to get good lighting, right distance and a level set up.
I would repeat the test a few times on this lens before you ship it back. Make sure it gets +15 or close to each time.

I assume you are using it with your 1D X and all of your lenses are not +15, just these two?

If you really get a +15 and you think it is right it is easy to test. Set up your 1D X with this lens at +15, set your camera on center focus and a wide aperture (at wide open or close to). Take a picture of the wife or kids eyes and head just a few feet from them and see if it hits a sharp crisp focus on their eyes. Close up that is when it is going to matter with this lens. At 10 or 15 feet your DOF has grown so large you may never notice. It is my opinion that close up is what matters with these lenses. The proof that it is really that far out is in the real life situation. If it isn't really +15 you are going to see it.

This lens is a tough one. I would check it a few times first then ship it.