I don't know how long you stay outside with her, but could you lengthen the time? Just stay outside till she goes? Or if she doesn't go outside, when you bring her in, keep her in an ex-pen or crated and take her back out every 15 minutes or so till she goes. Have a treat to give her whenever she does go outside. She'll learn pretty quickly.
I know that here it is too cold for my dogs to stay out more than a minute or two. It's been below zero and the wind chill is WAY below. Their feet start freezing pretty fast. So, they go out and pee but they come back in and go poo - usually on the potty pad. When it's warmer, I just stay out with them till they go. At times it has taken up to 20 minutes for them to decide to go.
Seems like they will always test me to see if I will let them back in before they go poo. Silly dogs.