This is a very common complaint, and one that may not require a call to the plumber. While there may be more than one reason the faucet is operating poorly, the most probable cause is sand, calcium or other build-up in the aerator. Simply unscrew it from the faucet head, noting the exact way all components fit together, rinse or use a soft brush to clean out the material that is blocking normal flow and screw the aerator back on. Soaking in vinegar is another great way to remove this build-up.
