I had a plumber rough in new CPVC piping and a single handle tub/shower faucet (Moen L2357). He left pipe stubs for the tub spout and showerhead. Both were capped off during the drywalling and tub surround process. Recently, I completed the attaching of the tub spout, handle and showerhead. When I turned on the faucet, the water pressure for both temperatures was fine from the tub spout. However, when I pulled the lever for the shower it takes quite awhile for even the trickle to start coming out of the showerhead (with any temperature).

I gather that something is clogged somewhere, but where? I checked and the showerhead itself is not clogged.

Short of busting a hole in the wall and replacing the faucet and/or piping, does anyone have any suggestions on how I can open up the pathway to the showerhead?