Is the leak actually from around the knob, or is it from the front vent hole under the ASA?
I don't think that you can really have a leak from around the knob unless air is leaking in to the front section of the ASA, and if that were to occur, air would instead leak from the front vent hole.
With that in mind, you need to concentrate on the o-ring inlet in to the front of the ASA insert. It could be dirty and need cleaning, or be worn and need replacing. A tiny bit of grease rubbed in to it to help it stay supple can also help.