Adding Ribbon Custom Action with Exposure Set to App-Web in Provider Hosted App does not Add any button to the SharePoint ribbon

Hi,

I am using SharePoint online E3 Trial tenant. I have a devsite and building up a Provider Hosted App to add Ribbon Custom Action for the document library. I am using Visual Studio 2013 Premium.

When I add the custom ribbon action with exposure set to "App Web", I could not see any ribbon button appearing. However, when I set it to "Host Web" the ribbon button appears correctly. I would like to understand why App Web exposed custom actions are not appearing in the ribbon?

Thanks