This may not be an issue with the plugin itself, but could be if you have limited session access or no session data allowed upon your site. It may also be because some other plugin can be disrupting the session.
The first thing I would try is to disable all other wordpress plugins, test the function of the store, then re-enable each plugin and keep testing the store. If this does not help, then try the following:
1) Check if your server's PHP config is set to allow sessions.
2) Check to make sure the session save path has been configured correctly. This is just a matter of adding a line similar to the following in the php.ini file:
session.save_path = /tmp
It is best to contact your hosting provider so they can do this for you. All you have to do is request them to fix the PHP session issue on your server and they should know what to do.
More information on sessions can be found here: