consists of regulating water from hot and cold sources. You may have individual valve control (separate hot and cold knobs) or just one knob (a mixer). Either way, the valve internals can be accessed after the water main is shut off for repair (new seats, seals etc.) and access of the piping to check for blockage. Mixer valves can get stuff stuck in them or misaligned parts that can restrict flow. Your new shower head may have a restrictor (low flow economy). If you aren't comfortable taking your plumbing apart, this is something that should be correctable without accessing the walls. Ask around, there is someone you know capable of this simple troubleshooting. Luck, Kizmet. BTW I'm not neglecting the holiday. Playing on the computer while everyone else is watching football. When Grandbaby wakes from her nap and dinner is ready, I will give it my full attention)