BEFORE REPLACING HEAD, make sure something isnt pinching the line or restricting flow. I had the same problem on newer unit and finally discovered i had moved the brute can over a line and then filled with water. Which the weight on line caused it to became partially pinched and had problem such as yours. If u want to test head, just swap them and see if problem follows head or stays on same side. If stays on same aide then u know it something else besides the head itself.