Good Morning C.. Yes it happens to alot of children. They are relaxed and not thinking about it. Went through the same with our now 3 yr old gr son. We tried about everything also, bribes, toys, reading books while he sat there. Nothing would happen until nap time and his pull up would be loaded.
His Ped said not to worry about it as it would just happen one day and it basically has. He still has a loaded pull up some days but that seems to be only when he has been a tad bit constipated. He has very large bowel movements some days.
What has worked for Him anyway, is when we told him he had to be a big boy and go poop in the toilet to be able to go to School in the fall. He really started trying then.
Now when he goes poop we do high 5's call his momma and daddy at work real quick and they give him Good Boy, proud of you's. Extra hugs when they get home.
Give her time she will come around soon.
God Bless
K. aka Nana of 5 gr kids
Benton, Ks