Well, it may be that the Wellness doesn't suit her... no food suits every dog, after all.
Or it could be a side effect of too-fast switching. A general rule of thumb for switching foods is that you mix the old and new together, gradually increasing the amount of new food in the mix till that's all you're feeding. BUT you must hold off increasing the amount of new food if at any time the dog's stools start to become loose. And only resume increasing the amount after they've returned to normal for a few days.
If that may be a possible reason, then the best solution would probably just be to ride things out for another 2-3 weeks, and see if time/adjustment makes a difference.
Another thing to look at is the amount of food she's fed. The leading cause of loose stools is overfeeding. So if the amount she's getting is near the top or in excess of the Wellness guideline, then it is probably worth cutting it back a bit to see if that helps.
If neiter time nor reducing the amount of food helps at all, then it is probably reasonable to conclude that this particular food might not suit this individual puppy that well, and trying an alternative may be a better plan.