Reflow is your key word. Make sure your heat gun is getting each chip hot enough to melt solder. You can tell this by putting little pieces of solder on top of each chip. When it melts, you have to go just a little hotter to get the bottom of the chip, where the BGA has a fault.
Regular heat guns blow a large volume of air. A professional rework station can vary the airflow. This fast air will blow your tin foil around and can blow the molten solder off the chip and onto the board. I can say from personal experience that it doesn't stick, but try to avoid that. I've only done one reflow, but if I did another I would partially tape over the intake on my heat gun to restrict the air flow. As long as the plastic of the gun doesn't melt, it should be okay. And if it does, it's only a $15 gun.