Firstly, you should ensure that they have access to the subscriptions/purchases you have shared with them. On their iPhones, go to Settings > [Your name] > Family Sharing > [Your name] (Me) > Subscriptions. This will show the shared subscriptions that they have access to. It is also important to note that only 6 members are allowed in a Family Sharing group, and you must have bought the Family or Premier plan on Apple One, so if you are in excess of this number, that is perhaps why. If you are under 6 in number, and your family members all have access to the necessary subscriptions, they can go ahead and cancel their own subscriptions by going (on their iPhones) to Settings > [Your name] > Subscriptions. This will allow them the rest of their most recently paid month on their own subscription before (unnoticeably) transferring to your version of the subscription shared with them. If the subscription is not currently shared with them, then ensure that under Settings > [Your name] > Subscriptions, the "Share with Family" option is checked, although this is turned on by default for all eligible subscriptions. I hope this has helped. Let me know how you get on following these instructions.