We have a old coal boiler which had been converted to oil and now natural gas which still works very well, except the last two years the radiator in the bath room has not gotten any heat and this winter the radiator in one bed room stopped getting heat . Both radiator have water in them as I drain the system every Spring and refill in late September early October. It is a gravity system. I have checked and the valves are open. What could be the problem as all the other radiators get hot when heat is on.