Twist-Out Definition Blues

I have a question for my fellow 4's out there (especially 4c's). When I do a flat twist 2-strand set, the top of my head stays relatively okay, but my sides shrink up and get spongy (if that's a word).

To give full disclosure, I don't have a good twisting technique. I don't know how to flat twist my hair. The top of my head stays nice and tight. But I can never get the bottom tight enough.

I don't know if the loss of curl definition on my sides is due to a different texture of hair, or my lack of experience with twisting my hair.

HELP ME!!! What can I use to keep my curl definition on the sides of my hair when doing a twist out? Is there any hope, or do I just need to live with it?