Hi Randomalbumname,
Thanks for reaching out using Apple Support Communities. Here to help.
We understand one of your family members is not seeing the shared storage benefit of your family sharing group.
We see you came from the related article, 'Add a family member to your subscriptions from Apple'. It sounds like the family members are already added to your Family Sharing group, so lets take a look at more specific steps to ensure iCloud is being shared with this family member.
Share an iCloud storage plan with your family
From the above article, we would specifically had the family member check that they have opted into the Family Storage using the following steps:
"Go to Settings > [your name].
Tap Family Sharing, then tap iCloud Storage.
Tap Use Family Storage."
If they enable this setting, they should then have access to the shared storage plan.
Hope this helps! Take care for now.