Shower heads often spray unevenly because their tiny holes have gotten plugged with mineral deposits. In order for water to flow freely, these deposits need to be removed. To do that, you will want to soak the shower head 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.