This has got me stumped.
My Sigma 15-30mm lens on my 10D stops down and remains that way after a few shots.
It doesn;t appear to be the camera as none of the other lenses behave like this.
I have totally reformatted everything - cf card, camera, took out camera 'internal clock' batt and put back but same thing still.
The only thing that helps is if I switch off the cam, take off the lens, replace with another lens and shoot, then switch back to the Sigma lens...after awhile. When I first take the Sigma lens off, the apeture remains closed. Sometimes, I have to take off-switch-wait-put back on-take off a few times then it works. Also, sometimes switching the lens between AF and MF helps to get the lens back into working order.
Any ideas as to what is happening, and what I should do?