There are at least a dozen possible reasons for over heating. Some you can check yourself easily, some you can't.
1. interior of radiator could be partially plugged up, even if you have good coolant.
2. Radiator could be loaded up with bugs and/or dirt and other crud.
3. Something could be blocking air flow through the grille.
4. electric cooling fan/s not working.
5. Belt driven fan has loose belt.
6. Bad cap leaking coolant
7. bad head gasket.
to name only a FEW