Attached is a picture of my shower valve. When I turn on the house water only the hot come out. It used to work but it hasn't been used in over a year and it's clogged. Before I call the plumber is there a way to unclog it? Perhaps the screw farthest right? The collar comes off but the screw is too tight for me to easily move. What is the round valve to the left of the screw? It looks to be 1.5 to 2 inches deep.