Cause
In most cases this is due to the upload permissions set for the module in the Shortcode Builder. By default, visitors/non-logged in users will not be able to upload files via the plugin. The module will not appear on the page for those users.
Solution
To allow anyone to upload files via the plugin as well, add the 'Anonymous user' role or the 'Everyone' role to the Who can see the module and Who can upload settings in your module configuration: