Iris control goes in and out

I just got the Tokina 11-16 for my BMCC and I'm already noticing the infinity focus problem that was originally discovered a while ago. But I'm also noticing something else that I'm not sure if I'd be my lens, or an error in the camera. It looks like if I press the iris button on something bright, it knocks everything dark but then I no longer have iris control. I can't use my back and forward arrows to manually move it, and the iris button doesn't do anything at all.

What's my next step?

If it's not happening on any of your other lenses send that lens back and ask for it to be exchanged for another. I just got version I of that lens, secondhand to boot, and it's working perfectly on my bmcc.

I suspect that when it comes to that lens they are not all created equally. Some just seem to suck. Luck of the draw I think.