6 weeks is to to early, she hasn't been properly socialized.
I place my pups at 9 -10 weeks. My new families never have a problem.
6 weeks they still need their mommy.
Since you have her now, it just will take time.
The only excuse to return a dog is health reasons.
One should understand that just because things are not working out at the time.
Does not mean for one to get rid of the animal.
She is younger than the other dog you had, so it will take longer to adjust.