Are you using smoothing products? - From the shampoo, and conditioner to serums and hair creams. They should all be formulated to help you smooth your hair.
Does your hot iron get hot enough? I used to have a flat iron that did little to straighten my hair. My newer irons work well.
When you use your iron, start at the root area and pull it smoothly and slowly (not too slow) down the length of the hair. Take small sections of hair and be patient. Don't hurry it.