Take some CLR (Calcium.Lime. Rust Remover comes in a gray bottle at walmart or similar store.) put it in a small bowl and hold it up to the shower head soaking it for a few minuets, it should do the trick.
Calcium deposits in the shower head can disrupt the flow of water. It is very easy to clean shower heads at home. You just need a few products and instruments. Step by step process is given at the following link: