Can you verify which version of the plugin you are running? This issue was addressed a few versions ago when private designs were first supported.
Be sure you have the most current version.
As a typical debugging method, try to use a default theme with your other plugins disabled to verify that the behavior isn’t caused by any conflicts with a plugin or with a custom theme that is trying to affect the shopping cart.
If you want you can paste your WooCommerce system dianostic here. Your version of WooCommerce may also be a factor.
If you can send me a link to you example, I can have a look at it.