There might air trapped in the cooling system. Try this.
Get the engine up to operating temperature. With the the engine idling slightly loosen the upper radiator hose clamp. Take a flat bladed screwdriver and slip this between the upper radiator hose and the hose neck on the radiator. Allow the engine to idle until all the air is purged out of the cooling system. Retighten the hose clamp.
Tester