You may have noticed a difference in water pressure between your cold and hot tap.
Generally, the hot water pressure is lower than the cold water pressure. This is usually because the hot water tank is lower in the house or the pipe run is longer. Cold water is supplied from the mains and is affected by mains water pressure e.g. a gravity-fed system.
Problems with your mains supply could be due to a fault outside of your property. Contact your local water authority to find out. It may be that your hot water is fed from a gravity-fed hot water tank whilst the cold water is fed directly from the mains supply.
Poorly designed shower valves and fittings restrict the flow of water. Such fittings need more pressure to drive the water flow out of the shower head.
In gravity-fed systems, there may be insufficient distance for the water to fall from the cold water tank to the shower. This lack of distance prevents pressure being created.