Certain solders melt very slowly and you will need a lot of heat. Clean the tip and add some flux. Then put some solder braid in-between the tip and the solder you want to remove. This should remove the solder into the braid and the wire should be free. Then repeat.
Losi 8ight-E 2.0, LRP Shark, Xray M18T custom extended, Traxxas VXL Revo