After replacing the thermostat, what else could make the engine overheat?

If you do not have proper circulation, you can have very erratic fan operation. I don't mean that the water pump is leaking. I am saying that the actual impeller( the blade looking part on the inside) may be damaged causing the coolant not to flow through the engine. One symptom may be 1 radiator hose hot while the other noticeably cooler