The problem could be enuresis.
http://familydoctor.org/online/famdocen/home/children/parents/toilet/366.html
I personally think pull-ups are a marketing ploy, overpriced, and give a child a sense that they can potty and still be somewhat comfortable! If you don't think your child has enuresis, then have you considered big-boy/girl undies and the plastic covers to go over? This way, there will be more of an uncomfortable sensation.
I knew a guy that had issues with enuresis from the time he was a child up until current age. He's now in his 20's.