Note whether the sprayer makes a knocking sound while operating. If it does, the problem is either a worn spray tip or a weak outlet valve. Replace the spray tip. If it continues to lose pressure, replace the outlet valve.
If the sprayer does not make a knocking sound, check the outlet valve. Use a 3/4" or adjustable wrench to remove it. Using a pen, press against the ball at the bottom of the valve. This ball should move no more than 1/8". If it moves easily and/or further than 1/8", replace the valve. If the outlet valve passes this test, replace the inlet valve.