It is CRITICAL for fine soldering work that your soldering iron tip be (1) clean, and (2) "tinned" before each soldering operation. It's all about rapid/efficient heat transfer between the tip and the substrate. An oxidized or dirty tip won't transfer heat well, but even a clean tip won't work well without a coating of fresh solder to transmit the heat between the two surfaces. The tip should be a shiny silver before each use. As soon as the tip surface dulls or discolors, it's time to clean & re-tin.