Did you squeeze the radiator hoses and push them to the lowest possible position?
If there is an air trap somewhere, that may help.
I was able to help a similar situation by putting a pressure tester on the cooling system over flow tank and pump some pressure but not let an air bubble into the hoses. When I loosened the hose clamp at the highest point possible in the system, a bit of coolant came out and then it passed wind and I had heat.