I have the SC 150 and the only way to get it to work is stay on top of it until it starts working then you are good to go.
Set it up and turn it on. Adjust air knob to no air in at all. Then slowly turn knob to add air. Turn until you see air bubbles coming into tank.
Then Slowly, very slowly, as slow as you can, (you get it slow) turn knob back until the bubbles stop coming into tank. This may have to be adjusted everyday until the foam starts to rise in the top. Once it starts working it will amaze you the amount of gunk it will pull.
Be patient it will work shortly. Mine took about 3 weeks.