even if you see the solder is melting (on top), but its not necessarily the case on the bottom (pcb), so in soldering process, heating up the solder to melting point only is not enough (on visible top), you need to heat up the base (pcb) as well to the melting point. once you get a melted solder at least in one plan inside the solder blob, then you can pull out that particlar point, but in bga case, you need to make sure the process above will reach into the centre joint which will receive the heat last. so in order to achieve that you need either two step 1) high temp. gradient (hi gun temp setting, which could damage the chip) or 2) use lower temp setting and wait long enough (patient) until all the portion is in temp equilibrium.