My Conley lens is operated with just air pressure (using a bulb) going into the shutter so possibly yours is the same. If that (on the upper lefthand side of the lens looking from the front) lever does not work, you may have to get it checked up. It sounds as if it works as it should on B, right? I suppose these shutters are relatively easy to get working, as mine still works fine after something like 90-100 years...
A bulb release shouldn't be too hard to find.
Another, short-term solution, is to use B and figure out some way to keep the lens open while focussing. A small piece of string, perhaps?