We have 3.5 baths. Water heats up in every faucet/shower/tub except for tub/shower in guest bath (guest bath is on 2nd flr of home, as is main bath; water heater is in basement, 2 floors below). Water in tub/shower in question is Luke warm at best and frequently shifts from warm to cold. Faucets in same bath produce hot water with ease. What likely is problem? What is solution?