Flat lower back

I have trouble getting my spine to curve so my tuck positions always look flat. It probably wouldn't be a big deal except that as I get older pulling into a tight tuck starts to bother my hip flexors (probably from the lordosis and inability to round my back). Pilates has been helping this and so has stretching. Here I'm doing a stomach drop, tuck, stomach drop combination and for me, this is a very "rounded" position.