J.N.
Being able to tinkle in the potty doesn't automatically mean she is able to control the poopies yet. There are different muscles and different nerve systems involved. It may be that she doesn't realize she needs to go until she actually is going (there is a point evacuation is reflexive; as adults we have learned to control it a whole lot more). It actually sounds a lot like that, since she agrees and says okay she'll go in the potty, but then never does. I would say give it a while longer, she may just need a bit more time to coordinate nerves, muscles, etc.