Whats wrong with 'Big Boy' undies

We have been having so much trouble with Shea choosing a pull-up over big boy undies. He uses the potty fine when he has to go pee, but we are struggling with #2, GROSS I know. Jace was the easiest kid to train. He was fully trained at 2 1/2 and never had accidents, I guess he was a rare case. I am sure Shea will decide to use his big boy undies full time soon, but if anyone has any good ideas until then let me know.