I've known a non-pooping dogs myself; it's my personal theory that some dogs just don't like to poop.
My great-great aunt Barbara had a springer spaniel that would eat and eat and eat...and poop about every other day, a huge pile, but only about 3-4 times a week. He would walk away from it as though he'd never seen it before, and the poop's presence was offensive to him. The vet couldn't explain it, the dog was perfectly healthy, lived to be 13 years old, and his whole life, never pooped more than 3-4 times a week!
But consider that maybe she's not pooping because something is hurting her when she does. When my sister's collie, Zach, first started having problems, she noticed that he wasn't pooping as often; it turned out that he had impacted anal glands. The vet told her that it must have been agony for him to poo. After his glands were evacuated (she had it done by the vet tech!) he went back to his normal 2 poop a day schedule.
I know she's only a puppy, but maybe there's something uncomfortable going on for her.