There’s probably air trapped in the cooling system.
Try this.
Get the engine up to operating temperature.
While the engine is idling, loosen the upper radiator hose clamp and slip a small flat bladed screwdriver between the upper radiator hose and the hose neck on the radiator.
Allow the engine to idle until all the air is purged out and nothing but coolant comes out.
Remove the screwdriver and tighten the hose clamp.
Tester