Couldn't find a lot to help you out and you might want to check out the official site as it seems you are not the only one with these problems. Maybe this site will help.
18 Pokémon GO Problems, Server Issues & How to Fix Them
I have Android Lollipop 5.1 and wanted to play Pokemon GO, only to realize that when I launch it, it goes to my home screen. How do I get it to work?