Hello user4,
Thank you for your inquiry. When a solder pencil heats up but the tip does not, this usually indicates the tip is oxidized, incorrect, or loose. Depending on the age of the pen, it could be the inside of the pen which then may need to be cleaned or replaced.
See below for for soldering iron tip longevity:
https://forum.digikey.com/t/hand-soldering-guidelines-for-tip-longevity/4032