I can’t say I’ve ever had this problem. Occasionally, it’s stubborn, I guess. Have you tried ironing it wet? What about starch? That stuff can work miracles.
The only time I can’t remove a crease is when I’m letting out a hem. You know how the Fairy Tale dress has a 4″ double folded hem? That is apparently standard in heirloom garments so you can let it down. So if I let out the hem or a dress that’s been worn and washed and ironed for a year with then hem in place, I can’t remove that crease. If you browse the topic of heirloom sewing, you’ll find all kinds of ways to cover up that crease. Ribbons, embroidery, lace inserts, fancy stitches, etc. Maybe that would help?
If the fabric is left with the ironed crease for a long time, that might affect how hard it is to remove.