Not everyone brings up obvious mucus. The purpose of airway clearance is not really to bring it up (unless you have a lot.) It is to thin and mobilize it in a way that your damaged airways cannot on their own. If your lungs sound clear with a stethoscope, there probably isn't much down there. You probably are bringing some up throughout the day and swallowing it without really realizing what is happening. When I am healthy, I doubt I bring up a teaspoon of mucus no matter what I try. But I am exercising the airways to keep them as healthy as possible, and when I am ill, boy does the junk come out! So airway clearance is really a long term maintenance therapy, not a competition to see how much mucus you can bring up.
So do a basic lung clearance every day for 15-20 minutes - you can alternate methods if you like - then get on with life!
Sue