Cant share iCloud storage with family
You need to contact Apple Support so they can "refresh your family members iCloud account". Dont tell them that specifically, but what can happen is that iCloud may not be acknowledging the change, and its something they can do on their end. 

Other questions for you to answer: 
Do you see that family member in family sharing? 
Is it only that family member? If not call Apple Support with the other parties present (physically or 3-way).
Do they belong to another family?