If the volume of water is restricted at the fixture, the most common cause is old pipes getting rusty and closing up. Iron oxide (rust) is formed inside the pipes when oxygen from the water combines with the iron of the metallic pipe. Iron oxide over time slowly decreases the inside diameter of the iron pipe and decreases the pipes wall thickness. Sooner or later (usually later) the pipe simply can not supply as much water as it used to. Remedy: Replace the pipes.