Hopefully it's either air in the system, iffy stat or header tank cap . As the the interior fans should blow hot if the water is circulating
Do the engine rad fans cut in ,is the bottom rad hose and flow /return to the heater matrix getting hot ?
Shortly before the rad fans cut in the thermostat should open allowing the water in the rad to flow through the engine , so the bottom rad hose should get hotter .
Heater matrix hoses are under the tmic and go through the bulkhead , if the water is circulating both flow from and return to block should get hot . If not could be stat or airlock restricting flow .