All my waterworks in my home work fine except the upstairs bath tap, I can hardly get much cold water from it, only a bit comes out, when I turn the tap towards off, it comes out a bit faster, but not much, weirdly enough the sink in the same room works fine as does the toilet, and other downstairs taps, so why can I hardly get anything from this one cold tap? how can I make the water come out faster, it takes years to run a bath