Responses
I think there's a possibility that the granddaughter is still too young and innocent to become one, so her amour levels just aren't enough yet. From what I've seen magical girls are of age 14+
Also, the granddaughter seems to vaguely remember her transformed granny in her dreams, meaning the possibilities aren't zero
So a annoying bratty child like her can become a magical girl but her sweet granddaughter can't ever become one thats just sad