OK, a few more questions to try and nail this down... does this happen with all apps? What apps, specifically? After you've let it try and install for awhile, try rebooting the phone, then check to see if the app is, in fact, in your app drawer.
(This happened on a tablet with me once...I was trying to install a game for my grandson and it seemed like it wasn't going to install, just kept loading and spinning. When I rebooted the tab to start over, it was on the homescreen, and worked.)