I have had this issue with desoldering capacitors off motherboards. The issue is, some PCB's have a thick copper grounding plate, and the moment you place your soldering iron to one of the pins, all of the heat gets wicked away immediately into the copper. Some even use lead free solder to make the situation even worse.
Here's what you do! Make sure you have a 60 watt soldering iron, Lead Tin solder, and a heat gun (I use a 1500W one). Heat the motherboard with the heat gun set to low, and move the heat gun over the board in an even pattern moving rapidly. The tip of the heat gun should be approx 3in away from the board. Sadly, I did not measure the specific heat in which I heat my boards to, but I typically heat them to the point where you can only stand to touch them for one second. Typically, at least on motherboards, this takes up to 10 minutes of heating (your arm will be very tired).
After the board is heated up, and the tip of your soldering iron is tinned with a nice large bead of solder, see if you can get the solder on the board to melt. If not, you either need to swap out a larger tip on your soldering iron, or heat the board even higher with your heat gun.
The main idea with this method is to get that copper ground plate saturated with thermal energy, so your soldering iron will have less work to do.
Hope this helps!