Question– “I felt the radiators around my house, and some were cold at the top and hot at the bottom. I was told I needed to bleed my radiators of the air inside them, to do this I opened the top of the radiator with a key, listened for a hissing sound and waited for water to come out. Some of the radiators hissed, and water came out… others hissed, and no water came out – and those radiators remain in the same state. Can you help?” 

Answer In our opinion, you need to go back and do it again. The reason is because you have to wait for the pressure to build up and then go back and do it again. You may have to do this many times.