This issue might occur because of

 - You may have a system custom master page that some placeholders were deleted or its visibility set to false.
 - The app installation service timer job is disabled.
and to solve this issue check this useful link

[https://blog.devoworx.net/2015/10/17/your-apps-stuck-loading-in-sharepoint-2013/][1]


  [1]: http://This%20issue%20might%20occur%20because%20of%20%20You%20may%20have%20a%20system%20custom%20master%20page%20that%20some%20placeholders%20were%20deleted%20or%20its%20visibility%20set%20to%20false.%20The%20app%20installation%20service%20timer%20job%20is%20disabled.%20and%20to%20solve%20this%20issue%20check%20this%20%20https://blog.devoworx.net/2015/10/17/your-apps-stuck-loading-in-sharepoint-2013/