When your workbook is opened on the customer's PC LockXLS stores some registration information. If you install new version of your software, old registration information is used. So, if your customer purchased 1 year license, and has 2 months to expiration, new workbook will work 2 months and then ask for new Activation Code.
In some cases you may need to install newer version of your workbook as a new project. You should not recreate your project and reenter all settings for it. Tools | Reset Project command will make your project as new. All settings are saved, but when files from this project will be installed on customer's PC, they will work as new project and ignore all previous activations.