I am working on a site that uses a lot of horizontal field groups. I am having a problem where any fields that sit in a field group and have "unlimited" set for the number of entries is unable to use the "Add another item" button. The AJAX call is returning some JS settings but no HTML.
The fields themselves range from simple text fields to node references. If a field is not residing in a field group then it works fine.
I will point out that I am using Drupal 7 and field_group version 7.x-1.1.
It seems that the Drupal core is unable to properly identify which button was clicked and so has not idea how to respond.