Why does solder not flow evenly on a brand new tip?
When a soldering iron has been left on for more than 30 minutes with no work done, it starts to get oxidized and turns black. This occurs even if the tip is brand new.
The oxidation of the tip forms a layer over the tinned surface (surface that has been coated with solder), preventing the solder wire from melting and wetting properly. Instead, the oxidized tip causes the molten solder to be repelled and ball up due to surface tension.
To prevent this, make sure to feed a generous amount of solder onto the tip once the soldering iron is turned on. Let the solder melt and coat the tip - this will ensure that wetting performance is maintained.
In the event that the tip has already been oxidized, you will need to clean and remove the oxidation. Check out the links below on how to prevent or remove oxidation!