Once you get those two knobs to move, with the top off watch what and how they move things, Tri-Flow, Tri-Flow, Tri-Flow. Then clean, clean, clean all the moving parts. Especially the two vertical shafts the selectors move up and down on and the latches. If these are gummy the selectors will bind.
I got two of these critters and have had to thoroughly clean them both.
Joe