Building core strength post partum

Hello ladies. I'm hoping some of the mommies here may be able to impart some advice! As the title says, I want to make a start on rebuilding core strength after pregnancy. My little man is now 9 months old and although since I began poling 5 months ago I have seem a huge improvement in the strength of my arms/legs, my core is WEAK. Inverts are a no-no for me at present, as I can't seem to lift my lower body higher than a tuck, and I can only hold that for a second or two! It's very frustrating as it's preventing me from progressing. Everywhere else on my body, muscles seem to be getting stronger and a little more toned but I feel my mid-section is just not getting a workout. I need it to catch up to the rest of me! Has anyone else experienced something similar? Do those pesky core muscles just take longer to strengthen than other areas?