This is a frequent question, and it is because you have a group account. You created a sub-user account and allocated all your download bytes to your sub-user, leaving your own account with 0 download bytes.
Please logon www.drivehq.com, click on Group Admin link, then select the sub-user and click on "Set Storage". Once you reduce the storage and download bytes allocated to your sub-user(s), your account storage and download bytes will increase.
Free service account has a small limit on storage space and download bytes, upgrading to our premium service is recommended.