Hi look at the level of water in your cold water storage tank probably not filling enough ball clock might need ajusting or fit a larger tank if required. Cold tank usually. Situated in loft normally they are galvanised steel or modern ones black plastic.if the tank is filling up to slow or not filling enough that is your problem, when you run a hot tap it will only run till your cold tank is empty then it will stop an easy fix for a good plumber. Hope this helps Alistair Ewen. A and m bathrooms.