Has she been checked for parasites? You should take your kitten to the vet at this point; she’s at the age where a vet should take a look. Is she still weaning, because she should be? Take kitty to the vet, they’ll give her a once over to make sure she is healthy and disease free.
It’s common for a six week old kitten to have a soft stool. At that age, she’s still adjusting to changes in diet. But just in case, she definitely needs to have a fecal test to check for intestinal parasites. She should should now be able to handle a high quality dry kitten food and she’s due for her first vaccines. Schedule her spay for around 4 to 6 months of age.