Ok kids, here's the deal.....
If it has a removable / replaceable tip, over time the heating and cooling cycles can cause it to loosen slightly eliminating it's soldering capabilities. Also, scale can build up depending on the quality of metal, length of time left on, etc. reducing heat conducting ability.
Here's how to fix it......
Take the tip off of the element. Take steel wool to the threads and replace the tip, or clean the threads of the tip and re-use. A little wd-40 will help on that first re-heating.