Issues of this kind are common when there is a third-party security software running on your computer (like an antivirus or all-in-one protection solution). This software place other applications, including our RAM Disk, in a so-called sandbox, isolated from the rest of the system. Being in a sandbox results in "forgetfulness" of the sandboxed app, as it is simply prevented from making and saving any changes, including the entered licence key.
Please check if there is a list of trusted applications or a white-list in your security software and add the RAM Disk app to it. If you are not sure where to find the required settings, please let me know what security software you have installed and we'll take a look at it.