Stop-Down issue with extension tubes

I can put a lens on my K7 and take it off the 'A' position and it will stop-down when I press the 'green' button or operate the DoF preview, in manual mode, and it stops down when taking a picture.

When I put it on the extension tubes it won't stop-down anymore, I thought there may be an issue with the tubes, but when I try them on my K10D and istDS it works as I would expect, stopping down to what ever aperture I set.

Am I missing something withe the K7 ?