Even a new tip will start to oxidize and solder will stop wetting if the iron is left on for 30mins or more
After iron has been
left on for 30 min
Tip temperature is 470°C
|New tip||Tip turns black due to oxidation
Solder will not wet to the tip in this state even if the tip is new.
Turn the power on, wait until the solder starts to melt, and then feed a generous amount of solder to coat tip. If you then leave the iron as it is, you should be able to maintain wetting performance like that shown in the photo.