- thermostat in backwards (rare, happens)
- still have a leak, air in system
- water pump failure (rare, happens)
- head gasket failure from prior overheat
to find the leak consider getting some coolant dye and borrowing or buying a radiator pressure test kit, then follow those instructions... as for blockage, it may be that debris from some failed component is causing blockage, or minerals if you used tap water to blend/top off your coolant (always use water without minerals such as distilled, or dehumidifier condensate, or some such).