Like Tim Taylor says "More Power" - in this case, more heat. For big jobs, use a torch. They are inexpensive in any home store, a regular butane one is plenty hot enough. Don't heat the metal so much it discolors; with a torch you could heat it cherry red if you wanted.

If you have an electric pistol-grip-with-trigger type gun (looks like an electric drill) make sure the heating element is clean and shiny too - oxidation on that will keep your wire from getting hot enough, and transferring that heat to your work. A pencil-type iron will only be good for electrical work

Make your wire and/or copper wrap clean and shiny with 400 sandpaper, a little flux (I use a brand called "Sta-Clean" - comes in a little white bottle - sold with silver solder, but the common plumbing paste flux works ok too) and you should be good to go.