This is typically the case for users who have a Single User License registered on multiple Machines. The reason this is the case, is that the Single User License is valid for one machine to present at a time. If more than one machine is registered to a single license, the watermark will show on all consecutive ProPresenter installs that are opened.
In most cases if you know which computer has ProPresenter on it that is opened, just close the application on that machine as well as the machine that you intend on presenting from. Once this is done, you can relaunch the application on the presenting machine and it will show without the watermark.
If you don't know which machine has the Unlock Code on it, you can install Bonjour Browser. This tool will tell you which IP address has the Unlock Code installed on it. Simply Open Bonjour Browser and select the dropdown arrow next to "_propresenter6" and this will show the IP address.
Bonjour Browser can be downloaded here.