If it's actual poop that's been on there a while, I'd soak it in cold. I know that seems odd but cold really does better at removing the poop than warm or hot water. Back when I worked in health care I knew why but now I forget. So give it a soak, then drain and rinse in cold and wash again. Hopefully that will take care of it. Good luck!
I had this problem with my daughter's fuzzi bunz. I got a shower sprayer and it worked great. I use to get so frustrated when I would take them out of the washing machine and there would still be poop stuck in the leg elastic.