Wha tis the pressure? You can buy a pressure gauge that connects to a utility sink faucet or outside faucet.
Are you on city water or your own well? If on city water, do you have a pressure reducing valve?
If you are own your own well, what type pump do you have; jet above ground or submersible in the well?
Have you cleaned any hard water scale build up in the shower head?
Quality Water Associates