Water in cylinder not getting hot?

Similar question to one already posted. My system is vented indirect, and although the radiators are getting hot, the water in my HW Clinder is not. I can feel the pipe from the boiler at the entrance to the cylinder is hot, but the return pipe at the bottom of the cylinder is cold. This makes me think an air lock or similar is present in the coil within the cylinder, is this possible, and how do I eleminate it??