Modern lead-free solder needs higher temperature - 360 C should do. You must have solder and flux on the iron's tip, this means clean the tip immediate prior to soldering, tin the soldering iron and immediately apply it to the joint to be made and then immediately apply the solder to the tip and pad - fresh flux is always required on soldering.
Heat only conducts well when the iron has enough solder to bulge out a bit and press against the pad - and there must be flux to reduce the oxide coating and let the molten solder actually touch the pad. Then there is rapid heat transfer (at which point you are adding more solder to the joint and immediately removing the iron so it can cool.
Hot iron, use fresh flux, work fast, don't breath the fumes...