add an app in sharepoint 2013

Hi

I am trying to add an app to my SP 2013 intranet site. But when i click on "Add an App" the page keeps on loading and not showing the pop-up page to create document library , list or anything.

I checked all site collection features and found all necessary features are activated. Add app in manage site feature also activated. anyhow again i have deactivated and activated the feature. but still no help.

I have customized the master page to the site. So, the site is appearing as per my master page designed. Could it be problem or some specific feature i need to activate.

Please help !!

Thanks