Check the lower rad hose and see if it is kinked or coming apart inside, I have had that happen and took forever to figure out, because the outside looked good. If not that, then check to see if the water pump is flowing as you can do this with the upper rad hose off. Start the engine for a few seconds and see if it pushes water out the top thermostat housing, this is also one way to bleed the system.
Good luck