When potty-training, my 3 year old was the same way. She would pee in the potty, but wouldn't poop, even though she had done it before. For her, it was like she was terrified to do it. She would always hide under the dining room table and poop in her pull-up or underwear there. When I saw her going there, I'd try to stop her and put her on the potty; she'd hold it. If she couldn't hold it anymore she'd cry hysterically while sitting on the potty, holding as hard as she could. I would try to talk her thru it, calm her down, it didn't work. So one time as she was heading for the table I said, "here, lets put your potty under the table!" and she was hesitant, but curious, so we did it, and she pooped in it! It was still hard; as the poop was coming out she was scared to let it go, but she did it. I kept the potty there for a few days, and it took coaxing from me every time she had to go, for a few days, but then she was fine. Now she has no problems whatsoever. I think for some kids its just scary, and maybe something they want to have control over. I don't know how this can help you, but hopefully it will in some way. Good luck!