The trick is to keep soldering tip always wet from solder and only clean it just before you solder. Second, always when done soldering and you cool down you soldering iron, melt fresh solder on the soldering tip before it cools down.
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Probably the equipment or maybe how you're touching the part you want to heat.
Take a look at this video where Louis Rossmann explains which tips works and which ones don't and shows why.
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