When you bend metal it becomes work hardened, so if you just grab it and try to bend it back it will want to bend anywhere but where it bent originally, because it favours where it's softer rather than harder. You then end up with a double bend, not a straight.
The secret to working metal is to trammel it so that it's forced to bend where you want and not where it wants, but as every bend is unique, so is each repair job. There's usually an element of trial and error involved, but the important thing is to watch carefully, and if it's not going the way you want, stop, and try something else. If you're careful, you can often see what's going to happen before you've bent anything past its elastic limit, but if you do end up with a bend you don't want you can continue with the same techniques to get it out again.
My father could get dinks out of steel rims without any bother, but then, he was a professional sheet metal worker.....
