Shower heads can develop a low flow situation due to clogging, a defective or stuck bath tub spout diverter, or a worn or defective shower or mixing valve. Just as with the faucets, start with the easiest possible issue and work toward the more complex solutions.
The Shower Head
Remove the shower head by unthreading it from the pipe. It will turn to the left (counter-clockwise) to unthread. Use a cloth over the shower head to protect it from tool marks. With the shower head off, try the water to see if there is adequate flow. If there is not, the problem is likely to be a defective tub spout diverter (if equipped) or a worn or defective mixing valve.
If the water seems adequate with the shower head off, look inside the threaded port. Shower heads may also have a filter screen or flow restrictor in them that can be seen just inside. The screen will trap sand, mineral buildup and other debris. Rinse the filter screen off, and if it comes clean easily, then inspect the nozzles on the outside. If the holes seem clogged with white mineral scale, you can sometimes disassemble the shower head to gain access to the nozzles. You may be able to dislodge the mineral build up by gently poking with a safety pin. Mineral scale can also be dissolved by soaking the shower head in a 50/50 solution of white vinegar and water, or using a commercial descaling chemical such as CLR or Lime-Away (note that some plastic showerheads may be marred by strong descaling chemicals, so test the chemical in an inconspicuous place first.)
Bathtub Spout Diverters
Bathtub spout diverters can sometimes be repaired, but it is often easier to simply replace the entire bathtub spout. Look for the hex head, allen screw or phillips screw at the six o’clock position, and loosen it. The spout will then either unthread counter-clockwise or pull straight off the stub out pipe. You do not need to turn off the water in order to remove the bathtub spout. Take the entire spout with you to the plumbing department to help choose an exact replacement.
Diverters mounted in the wall come in several different configurations, and disassembly varies by make and model. While there are repair kits for many diverters, you do have to turn off the water supply to the shower. A common scenario is to have to turn off the water at the main supply entrance into the house.
Generally, you must remove the handle(s), then the decorative escutcheon plate, to reveal the diverter assembly. The diverter assembly will have screws, a nut, or a c-clip that holds the cartridge into the body of the valve. Remove the diverter and take all the parts, including the cover (if separate), to the plumbing department to get an exact replacement.
The article on Low Water Pressure for the whole house will have more information on looking for blockages in the main water supply lines. Usually, this results in low water pressure at every fixture.