Are you sure that you put enough coolant in, did you measure it?
It's possible you have an air lock! The rad looks full but it may not be a full system.
If you are still topping up and have no leaks my guess is an airlock.
The older days we'd have left the radiator cap off (cold engine) and started it up and as it warmed up and the thermostat opened the water pump would push the coolant around and would force any air out and the coolant level would drop again. Then top up again and repeat.
