The squeezing up of the chest to extend the thoracic spine will help to get the lumbar spine set if you think of it as a "wave" of extension that starts at the top and goes all the way to your ass. But you also have to know how to engage your lumbar erectors to begin with. Can you you do that? If not watch this for some guidance YouTube
Then, squeeze up harder to get the back flat. Harder than you think you need to. Like someone else said, if you're not uncomfortable, you're doing it wrong.