It’s time to change the heads.
Mine did the same thing until it stopped pumping all together.
Changed the heads, all better.
Word of advice... cut the zip ties and pull out and save the nipples and screw downs on the old heads just in case... trust me!
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.