For additional soundbanks to work inside the HardKick plugin, they need to be activated to your main HardKick license key.
When activated they are connected to your personal HardKick plugin license.
A sound bank can only be activated once.

In case you can’t find your original HardKick license key, click the settings button in the top right of HardKick.
You will see your license key in the top of the panel. Right click to copy the license key to your clipboard.

Contact Psylus Audio support in case you experience issues.