Propably you have not swtiched to your new license.
When you run FlashBack Pro for the first, it will automatically download a trial license. If you upgrade that trial license to a full license, it will automatically update when you next start FlashBack and the watermark will disappear from your recordings.
However, if you purchase a new Pro license instead of upgrading the trial license, you will need to change which license your app is using before the watermark disappears.
When you start FlashBack while it is using a trial license, a pop-up will appear which tells you how many days are remaining on your trial. Click the 'Enter full license key...' option on that pop-up and enter the new licence key that you will have recevied by email.