A lot of this depends on what type of iron you're using and how well your equipment was taken care of.
Solder needs a few things to melt (and adhere to the iron). A sufficient temperature is a start, but soldering is built on a few principles, not just getting things warm enough. The biggest reason that solder usually will not melt, or stick to an iron is oxidation on the tip. You'll want to make sure it's shiny, rather than dull, discolored, or burnt. This can also block or impede the transfer of heat.
Solder wants to melt and stick to more of itself, or anything copper. So you'll need to have a good clean surface for effective solder. If you don't have a means to clean your iron, [guide|175931|that's where I would start].
Many tips also have a hot spot where you'll be able to melt solder the most readily. Usually this is towards the end of the tip, but form factor and quality will affect this. A hook type tip will likely be hottest and the bend rather than the tip of the hook.