I agree with EmisAngel, our family subscription had expired due to out of date payment card. When we re-subscribed, all the family members lost their OneDrive quotas and OneDrive was reporting they had only 5GB of quota (it should be 1TB for M365 Family members).
We found the correct solution by ErmisAngel above and also by Karl Timmermans in this support thread for the same issue.
My wife, who is the subscription owner went to https://account.microsoft.com and then clicked to 'manage' her M365 Family Subscription.
She then turned off automatic renewal for the subscription (you have to ignore the attempts to get you to downgrade to a lower subscription and click on the scary sounding "I don't want to keep my benefits" button at the bottom of the page). She then clicked on the 'Turn on automatic renewal' button.
This seemed to force all the benefits of the subscription to be re-applied to all the family members. Everyone saw their OneDrive quota instantly change from 5GB to 1TB as expected. :-)