I am not sure why. Newbie question.

 From:  Rudy
Hello all,
Hello Michael. I think Michael is pretty busy today...

Can somebody explain to me this.

I want to boolean diff. the inside of my ring (using the cilinder), to get rid of the base of my rectangular head, so that the base of the head will follow the inside curve of the ring.
When I do the boolean operation (first select my base obj....ring), then the obj. to subtract (cylinder), the operation does not happen. the base of my head is still sticking down inside my ring.

What I do wrong?
Thank you in advance,