Showerheads often spray unevenly because their tiny holes have gotten plugged with mineral deposits. In order for water to flow freely, you need to remove these deposits. To do that, you may want to soak the showerhead in in vinegar. And that may mean removing the showerhead from the shower arm.
Newer showerheads screw onto the shower arm, the pipe that comes out of the shower wall. Older models feature a shower arm with a ball-shaped end that acts as a swivel for the showerhead.
If you can't remove your showerhead or simply want to skip that step, you can soak your showerhead by using a rubber band and a plastic bag. (Note: This method is best for showerheads made with chrome, stainless steel, or other protected metal surfaces.)
First slip a rubber band over the top of the showerhead. You may want to loop it around the shower arm once or twice so the plastic bag will stay in place. Then fill a plastic bag with white vinegar. Attach the bag to the showerhead by slipping the top of it underneath the rubber band. Wait one hour, then remove the bag and turn on the water to flush. Polish with a soft cloth.
If mineral deposits prove to be beyond the power of vinegar alone, you will need to remove the showerhead to do a more thorough cleaning.
Disconnect the showerhead. To disconnect the showerhead, unscrew the nut at the shower arm. Take care not to mar the fixture's finish. Use a wrench rather than pliers. Cushion your tool with a rag while you work.
Rinse the showerhead. Run a sharp blast of water through the showerhead by holding it upside down underneath a faucet. Your goal is to rinse loosened debris out through the opening that connects to the shower arm.
Dismantle and clean the showerhead. If there are still mineral deposits, you can scrub the showerhead with an old toothbrush and vinegar to loosen debris. Use a toothpick or safety pin to poke out additional deposits. Then soak the parts in vinegar overnight to dissolve any remaining deposits. Rinse again.
Note: Some showerheads boast pliable plastic nubbins that can be manipulated to break calcium deposits loose.
Reassemble and reinstall your showerhead. First wrap new plumbing tape around the threads of the shower arm to ensure a good seal. Reattach the showerhead to the shower arm using the wrench. Protect the fixture's finish with soft rags or towels while you're working.