As the previous post mentioned, flux removes the oxidation of the copper.
If it is a circuit board that you are sodering, try rubbing some steel wool lightly over the pads you are soldering too. It will expose fresh copper and the flux within the solder you are using should be sufficent. Clean the board before soldering, you do not want a loose piece of steel wool shorting out a trace. When soldering to larger component like switches, I'll use sand paper or a very light file to remove oxidation it oxidation is bad. I ran into one manufacture's cheap switches, the metal they were using was SO BAD, solder would not even flow with generous amounts of flux applied after cleaning with a file.
You can eventually tell when you do not have enough heat or oxidation is preventing the solder from flowing.
I use 12 gauge wire for a supply bus around my train layout, using Digital Command Control (DCC). Using plumbers flux is sometimes the only way I can get the oxidation removed from the wire so the soler will flow when I aply take off taps from the 12 g wire.