Your showerhead probably needs cleaning. Take a plastic sandwich bag and fill it with vinegar – any kind will do. Place the bag around the showerhead so that it is entirely submerged in the vinegar. Use a zip tie, or some string, or a large rubber band or a trash bag tie to hold the plastic bag securely in place. Leave it there for at least 24 hours. The vinegar will literally dissolve the mineral buildup that is causing your shower head to clog up. After the appointed time remove the bag and turn the shower on. You will be amazed at the improvement in water pressure. It should work as good as the day you originally installed it.