What mad solder type is used for BGA?
Thermal inertia is playing against you. Also take into account that lead-free solder needs temperatures in excess of 220°C to melt down (compared to 180°C for tin-lead solder), so the thermal gradient will be quite high to begin with.

Because of this, I would recommend preheating the board to 120°C by using one of the following methods:


A preheating plate, or
An oven


Then apply hot air to desolder the BGA chip.