It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

When I started playing coop it worked for the first level I played but now it never does. I have tried both hosting games for my brother and joining my brother's games after he invites me repeatedly and it never works.

What keeps happening is that the person who accepts the invite ends up in a game alone without the other person. The person hosting the game never sees the person they invited show up.