Hot air rework stations use a thermocouple in the airstream to accurately control the air temperature - its essential to use a temperature that won't cook the board, which often means you have to wait for a couple of minutes for the board to come up to the air temperature - trying to rush by increasing the air-temperature too high will lead to burning the board, so just be patient. Use the largest nozzle you can so that more of the board heats up and the airflow is less violent (small nozzles can lead to components being blown off the board!)
And use lots of flux, the components should be swimming in it.