Why is my sportage overheating?

2014 kia sportage. I changed the thermostat twice. The water pump seems to be fine. The car will push coolant into the reservoir but won’t pull it back into the radiator. If I fill the radiator with fluid it will stay cool for about 2.5 hours of driving, after which it gets hot. When it starts to get hot, I just put the heat on inside my car and it cools back off. It’s like it intermittently has enough coolant and then it doesn’t. I dunno man, it’s confusing the hell out of me