Add some 60-40 solder to the solder joint (which is likely lead free tin and difficult to work with because pure tin has a very sharp melting (freezing) point. Adding the 60-40 will make it easier to work and give you more mass in the joint so, once heated, it takes longer to cool so you have more time to suck it out.
Then, to heat your newly formed melt, use a 700° or 750° tip. The goal here is like making a rare steak with a crispy outside - high heat for a quick hit. If you can't get it melted and sucked in about 5 to 7 seconds, wait for it to cool and try again. Working quickly prevents the board from getting hot and (1) melting the glue that holds the copper to the PCB, and (worse), (2) oxidizing the copper to an oxide and ruining the pad and adjacent traces.
Use a tip that makes good heat contact like a clipped cone (not a point). Transferring heat quickly is the goal. Also, clean your solder tip to keep it oxide free. I prefer a damp sponge. Wipe and spin at the same time in a single quick pass (you should hear a faint hiss of steam but not char the sponge - then you know it's wet enough without being too wet. Let the tip regain its temp by waiting 5 to 10 seconds before attempting anything.
Good luck.