I suspect that the bar is an inch too far forward of your midfoot when you start. Your shoulders are too far ahead of the bar and your weight may be on your toes. This makes it more difficult to get the back flat.
Yes, you're not getting squeezed up/back flat enough.
Follow the setup in the book to get the bar over the midfoot (shins one inch from the bar). Then with stiff legs reach down to the bar. Keep your balance over the midfoot as you bend your knees to bring your shins to the bar. Then, squeeze your chest up between your arms as hard as you can.
Good luck!