It might have just been a dirty iron? I use a lot of spray starch and about once a month I need to clean the sole plate.
I've 'heard' you can pour some salt on a piece of waxed paper and iron over it but haven't tried it.
What I use is Dritz iron cleaner. comes in a tube like toothpaste.
I cut a 6" square of cotton batting, fold it up (3 layers,to keep the cleaner from going through to the ironing board cover) then squeeze a line of the iron cleaner out on the roll.
Drive the hot iron back and forth a few times, you'll be amazed at how dirty the cotton batt gets
Pitch the dirty batt and go back to pressing your brains out!