The following reasons are common:
1) Not waiting long enough for the credits to be added to your account. This process can take upwards of 5 seconds. The bottom frame must reload properly or credits will not be added. Predetermined time is a minimum of 10 seconds depending upon the account type.
2) Account might have been deactivated and/or suspended by administration due to
activity violating our terms of service.
3) Wrong surf-for-credits url.