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It could be a couple things, It could be plugged with Lime, Calcium, you name it. This is a easy fix, unscrew it from the pipe, screw on a new one. or Use CLR to clean it by dipping the shower head in a cup full.
It could be the diverter valve is not working, find at your local hardware store the brand you need and replace it.poetchaser38839.6814208681