A tub and shower combination uses a diverter to direct water toward the showerhead. If a substantial stream keeps running from the spout after you switch modes, the diverter may not be closing fully. That can reduce flow at the showerhead.
Check when the spout runs
Notice whether it runs only while the shower is on or continues after you close the main handle. Those patterns point to different parts of the fixture. Tell the plumber whether the diverter is a pull knob on the spout or a separate control on the wall.
Share photos before ordering parts
Photograph the controls and the spout without taking them apart. If the problem began after a spout replacement, mention it. A plumber can check how the spout fits the pipe and whether the diverter is part of the spout or the valve behind the wall.
Learn more about our home plumbing service.