I don't think length affects it all that much. When you blow dry your hair, are you blow drying it straight with a thick brush? If your hair is thick enough to hold a product, then I would recommend using them. Then of course top off with a flat iron.

I had this problem too, honestly the only way I found to solve it is to upgrade to a better flat iron.

I always use a straightening iron on my hair as opposed to the blow dryer, because A) I shower at nights and do my hair in the morning, and B) my hair takes FOREVER to blow dry. On a straightening iron it is extremely important to make sure that you have ceramic plates (usually a goldish color) as opposed to metal (which are usually silver). Metal straightening irons take forever to heat up and work horribly. I don't know why they make them.

Make sure to use a heat protectant before you straighten it, then go over the sections two or three times each. At the end, put on flexible hair spray or some kind of holding creme. Hopefully, that will help you a little more.

What I do- After I get out of the shower I use a wet-to-straight hair straightener to dry my hair. The point is to keep frizziness down, it doesn't have to be straight after using it, just dry. Then I use my chi straightener until my hair is pin point straight and perfectly dry. If I want it to be shiny, then I spray hair spray on a brush and brush it through. This is just what I do, but it might help. I have very wavy, frizzy hair.

The best styling product I can recommend is Phyto's hair relaxing balm. It's consistency is like a light gel, but it's MUCH healthier than anything I've ever used for your hair. Even if you use entirely too much, you'll never know! It keeps my hair frizz free and straight even through humidity and just about everything else, really. You can tell a difference right away and it makes you fall in love with Phyto :)

Also, try straightening smaller sections at a time. That way the heat's more concentrated, and if you've got a heat protectant (which you should) it'll make much more difference in the overall look.

your length could have something to do with it. my hair , when it was shoulder length, would always flip up later because my shoulders were pushing it up .

Maybe you're trying to straighten too much hair at one time? Therefore, you're not getting the full effect? Otherwise invest in a more expensive one!

Good straightener is key; at least for me.
Spend a little more money on a nice straightener and it will pay off.
My hair used to go wavy/curly within the hour, and now it stays straight all day, even without product.