I have had one of these for a couple of years and the other for alot longer. I would say they have had alot of use but after leaving them to heat up for about 10 minutes I find the solder is not adhereing to the joint. It behaves like toffee and is quite sticky, not fluid. Are my irons not heating up enough?
Tips aren’t clean, solder isn’t clean/right type.
Post a picture of the tips.
Post a picture or make a question clear to solve your issue.
In addition to the potential problems listed by skimask, are you sure that the solder you are using has a flux core? If not, you’ll need to add flux.