In order for your Sub-Accounts to create Flipbooks, they'll require Upload credits which will need to be allocated from your Master Upload credit pool.
You can also recoup Upload credits from your Sub-Accounts back to your Master Upload credit pool if you find you've over allocated to that Sub-Account or need them elsewhere. Follow the steps below and refer to the screenshot for clarity:
Step 1 - Choose a Sub-Account
Once selected, access the Sub-Accounts Account Settings/Profile page.
Step 2 - Click on Services
In the left menu of the Account Settings/Profile page, look under Services and click Upload Management.
Step 3 - Choose number of credits you want to transfer to Sub-Account or recoup back to Master
Enter the number of credits you want to transfer OR recoup, and click on the corresponding button.