Good tub pressure, Weak shower pressure

[LJW]

I have a tub/shower with 3 control knobs. The water pressure in the tub is strong, but when I turn the middle knob to divert the flow to the shower the resulting pressure from the showerhead is very weak. I've replaced the showerhead and seen no improvement. Any ideas ?

[Mike Swearingen]

Turn off the water, remove the diverter handle and replace the diverter. It isn't the showerhead.
Good luck!
Mike