Soldering tips must be replaced if they appear to not be working within split seconds. Even if the tip has been cleaned, sometimes they go bad due to a never ending stream of corrosion/oxidation, or other unknown reasons.

Tin the tip after using it. Never leave it sit there and cool off with just a dab of solder on it - there has to be a ball of it on it, and you have to clean the soldering iron after using. A sponge and water can work, or brass shavings (nesting). However, water has minerals in it and may corrode/oxidize the tip - consider distilled water or brass shavings. Brass shavings may scratch the tip, but appear to work better than water in some cases. So try out what is best.