My daughter went through something like this. She was dry at night and peeing in the potty for over a YEAR, but still asking for pull-ups to poop. She kept saying, "I can't! I can't" and I finally realized she really physically couldn't. She was struggling with painful pooping and needed to contort her body to make it happen, and she couldn't do that on the potty. We quickly started her on Miralax, and she was completely potty trained four days later. We continued the Miralax for a few months to give her intestines time to heal, and she has never needed it since.
ETA: I feel awful for having pushed her on it, though. She said she couldn't do it, and she meant what she said. I should have listened to her.
Rachel
Mama to Abby (5) and Ethan (2)
When you know better, you do better.
-Maya Angelou