There can be several reasons for this. Frequently, low pressure throughout the house may be an indication that the pressure regulator is mal-functioning or needs replacing. Often times if installed, a house filter may become clogged or a water softener may not be functioning correctly. If pressure is low in one particular sink, chances are that water saver or strainer located in the faucet, is clogged with sediment. On a larger scale there could be a water main break within your service area of fire department activity.