I agree with asking your vet the next time Luma takes a trip there.
I know Sophia went from 3.2 pounds at 9 weeks to 6 pounds at 20 weeks, but then again, she eats like a HOG. She is now 8 pounds but her body is still all skin and bones, like Luma's. Whereas Brody (who also eats like a hog).... has a bit more cushion and literally his arms are twice her size
. So I think dogs can have different body types, but she should still be gaining weight.