I replaced an old (very old!) upstairs radiator after draining the system. When I turned on the water again to test, the tank filled up with water, but no water has filtered down to any of the radiators. Ive tried opening up bleed valves - nothing happens and I also completely disconnected one side of the radiator - again nothing. Despite how this reads, I am quite proficient at DIY, but this time I'm stuck. Any ideas?