Place the chicken breasts, chicken broth, carrots, celery, onion, thyme, salt, and black pepper into the slow cooker. Cover and cook on low for 6 hours until the chicken is tender.
Remove the chicken from the slow cooker and shred it with two forks. Return the shredded chicken to the pot and stir to combine.
In a mixing bowl, stir together the flour, baking powder, milk, and melted butter to form a thick dumpling dough.
Drop spoonfuls of the dough directly onto the surface of the hot broth in the slow cooker. Avoid submerging the dumplings.
Cover and cook on high for 1 hour until the dumplings are cooked through and no longer doughy in the center.