I also have a Hot-air pencil type station, and in all honesty, of the 4-5 BGA's I've ever seriously tried to reflow, only 1 has ever worked.
I've tried graphics chipsets on Compaq/HP laptops and they just failed completely, possibly because of the sheer size of the silicon die. The one that worked was a small-ish chipset on a Wifi Dongle that I accidentally kicked out of the front of the computer.
The trick seems to be to warm up the whole board gently from cold, to probably 200 degrees Centrigrade over the space of a few minutes, then really close in with the hot air set to 350 degrees or so for 30-45 seconds absolute max, then turn down the heat to zero over a few minutes. If the board is small, make sure it is secure and can't move. Also, nothing will reflow without flux, a little poke with a screwdriver to shift the solder on the pads seems to help too.