Can we once again talk post-pregnancy stomachs

I've been working out with a trainer 2x a week for a few months - doing a lot of circuit training. I added in other exercises at home on other days. Just started running recently as well. I'm really working on strengthening my core after it was destroyed by 3 pregnancies I'm eating much healthier as well. The inches are falling off everywhere.

The trouble spot is, of course, my stomach. I can see the waist (the measurement they do above the belly button) getting smaller. But nothing is really happening below the belly button. Well, I can definitely suck in my stomach better. But the measurements are not really changing.

What am I doing wrong? I'm really trying hard to engage my core when I'm working out in addition to doing planks, etc.