The cause for this is almost always that you have an item on your TabBar that doesn't have an icon assigned to it. One easy way to find out which item it is is to go to your app dashboard and look at the Check For Errors box under Additional Resources. In the picture below, you see the Check For Errors box of an app that will not show in the Preview App. This is because they have a TabBar item that doesn't have an icon. The Check For Errors box tells them they need to go to the Views module, and edit the one called Content to give it an icon.
You may see other errors in this box as well, and it's always a good idea to fix anything you see in it. It can only help!