sometimes thermostats are f'd up from factory . you can check it by taking it out and putting it in a pot of hot water on the stove. if it opens then its probably good.
also you have to make sure that it is going in the right direction with the spring side facing into the block.
also flushing out the radiator is good but did you check if you were getting flow through to begin with?
also a lower temp thermostat will do the trick sometimes, as it will open sooner and let the coolant through- thus keeping it cooler. however, if you live in a cold climate you would have to change out the 'stat seasonally.