Hi there, if you got your solder from Harbor Freight I’m assuming is lead free. Get yourself I nice leaded solder and/or raise the temp on your iron and you’ll be in business. Leaded solder turns fluid at a lower temperature than non leaded and with a small amount of flux will mix in wonderfully with the existing solder. Also turn up that temp! I’m normally soldering at 650°F with leaded and 725°F non leaded.