Can anyone lend any advice on shower/tub repair?

I have a shower/tub combo with a problem that I can't seem to fix. There is not enough water pressure when I pull the diverter on the faucet to force water out of the shower head. The pressure in the rest of the bathroom is fine. I have checked the valves and replaced the faucet...what else should I check/replace?Can anyone lend any advice on shower/tub repair?
IT'S THOSE DAMN 1.2 GALLON SHOWER HEADS!


PULL OUT THE WATER REDUCER WASHER %26amp; DRILL IT OUT W/ A 3/8'; - 1/2 '; METAL BIT!





THANK YOUR ELECTED OFFICIALS!Can anyone lend any advice on shower/tub repair?
when you replaced the valves was that just the valve stems or whole valve body.some valve bodies have a small strainer inside the outlet to the shower,if this is the problem it can be fairly involved to remove this strainer(which i still have never learned the purpose for)very first thing ,take the shower head off the shower arm that comes out of wall.with head off,turn on shower,if flow is good clear debris from shower head strainer(this is a very common problem.if flow is not good you can check flow to valve by turning off water and removing valve stems.slip something through holes and over valve stem receptacles(pc. of pipe or tubing)just to keep too much from entering walls when water is turned on.hold pipe or tubing and have someone turn water on just for 2-3 seconds,this should be long enough to determine if water flow in is ok.if it is slow try to flush it out with longer bursts of water pressure.(this is fairly uncommon unless in a newer home where debris may have fallen into roughed-in water lines and now be showing up)if none of this is the cause,it probably goes back to the valve body diverter strainer which can only be accessed by going into the wall behind tub.(i've only ever encountered this once)if you did not replace shower head and arm i'll bet that's the problem.