I had the same issue, spent ages scratching my head and resetting every option. It was something soooo simple. In the action settings in the promotional pop up if you have not typed a text label in the 'Link Label' box no button will be visible. See my image below - type something in the box alongside your link and save - voila! The box appears. Hopefully you find this and don't go through the hours of frustration I spent on such a simple oversight!