You have said conflicting statements,are you saying that the boiler pressure is low in which case the pressure needs increasing by means of the filling loop which is a flexible pipe connected to the cold mains usually by the boiler,or is the cold water flow rate low which could be several reasons but assuming all stop cocks are open there is a regulator on the cold input to the boiler but water does not slow down without a reason
toptrish, January 2016