If you use it for a long time or leave it plugged in for a long time, the tip of the soldering iron will turn black and the solder will not stick at all.
The cause is oxidation
The tip of the soldering iron is plated with iron. When the soldering iron is new, the iron plating blends very well with the solder, so soldering can be done smoothly.
However, if the soldering iron has been used for a long time or left plugged in for a long time, the surface will oxidize and the solder will not blend at all, making it impossible to solder cleanly.
Revive in an instant
There are items that can restore the tip of a soldering iron to a shiny new condition. It's called a chip refresher or chemical paste.
I use the chip refresher BS-2.
How to use it
It's very easy to use. Insert the heated tip of the soldering iron into the tip refresher. 1 second is all it takes.
Then, clean the tip with tip cleaner.
I use a metal tip cleaner, like a steel scrubber, instead of a water sponge. I use a metal tip cleaner that looks like a steel scraper instead of a water sponge, because I don't have to soak the sponge with water, and because I don't use water, the temperature of the tip doesn't drop. By inserting the tip with a zap, the tip becomes very clean. It is too convenient and I recommend this as well.
The tip of the iron, which was black earlier, has become shiny.
It's also inexpensive, so you can always have it on hand to keep your tips in tip-top shape.
This is a good item to have on hand if your tip has lost its light and is not easily soldered.