The tip is badly oxidised, it requires force to melt the solder as the oxide provides a pretty large level of insulation, the force increases contact area, and the chance of penetrating the oxide layer, even if ever so slightly.
If you're super careful, you may be able to get away with gently rubbing it with standard copy paper until it removes some of the oxide (start at the furthest point from the tip, so if you stuff it, you ruin the part you never use), then, when you are through, your flux and solder should do the job to get rid of the rest.