Increasing water pressure in shower is beating me

Is it only the shower or every tap in the house? If it's just the shower, unscrew the head and soak it in vinegar overnight, scale blocks the holes more than people think. While it's off, look inside for a small plastic flow restrictor. And check the shut off valves for the shower are fully open, someone often half closes one during a repair and forgets. If the whole house is weak, check the main valve is fully open and whether you have a pressure regulator where the water comes in. Those can fail or be set low.