You are warming up a very large power or ground plane. Takes a lot of heat, well directed. The little bastahds like to spin, tip, dance, and roll. The soldering iron interferes with the overhang of the orange stuff. I was using the smallest tip available, so I squished it in the vise and made a thin shovel point out of it. I can now come in under the overhang, and tag the insert and even create a little downforce which greatly improves heat transfer. A solder miniscus is VERY hard to get consistent, and the heat needed to warm up the surrounding area to get it able to wet the solder is enough to darken the board if you stay in one place to long. If you move, you have to do it slowly so you don't leave a "Wake" as you leave. It's tough to keep a professional touch on it.
