remove the shower hose where it connects to the faucets. You will see a plastic part with a small hole on the faucet assembly. Remove this part temporarily. I just put a cup over it and turned on the water and it pops out. Underneath you will see a rubber washer of some sort with a hole in the center and a black piece of the washer covering the hole. Pull the black part off of the washer and reinstall. Then put the plastic piece back and finally your shower hose. Hopefully like us, you will get the shower pressure you seek.
I was getting ready to shower one day and like yours our pressure has not been the greatest. All of the sudden it was hardly spraying out of the shower head. I took a long journey of pulling hoses and cleaning screens before finding this part. My dealer could not tell me what function the removed part served. I have a gauge on the water inlet and it reads 40 psi. As advised in previous comments check the screens in the water line. Remember there are screen outside also.
good luck!