I've some problems with the push button on the back of my S&S MDX D300, but only using the Sigma 8-16. For this lens I use a zoom ring modified to fit the body lens, so the problem is link to the particular configuration. But I recognized that S&S have a very very little tolerance in the camera position inside the housing.
Have you removed the plastic rear display cover from the camera body, and the rubber eyepiece too?
This is very important to make a correct "accommodation" of the camera inside the housing.
I agree with the latter part " Have you removed the plastic rear display cover from the camera body, and the rubber eyepiece too". I never had any issue with my S&S housing but from the beginning i removed the plastic cover and the eye-piece.