As Corrina said, it's hair properties that count. Here's a link to help you with that Live Curly Live Free - Curly Hair Basics The best thing you can do is read, read and read some more. There are stickies at the top of each forum on nc.com that will help you learn what's going on.
Aside from the flat iron damage, the canopy of the hair is usually less curly because it's the most exposed (to the atmosphere: wind, etc.) and therefore dryer and more damage then the under layers.
As you learn to take better care of your hair, the curls will come back. You may want to get a haircut that's "geared" to wearing your hair curly--long layers, etc., to "lighten" it up and let it curl/clump better.
You want to pamper your hair--lots of protein and moisture. It won't fix the damage, but it will let your hair be the best it can be until the damage is removed (cut off.)
There's a lot to learn on nc.com. Good luck on your curly journey.