Invites, right now, can show up in a few different areas in Facebook when being received. They can be in the top 'jewel' notification area. The also can be in the left-hand side bar in the "Apps" section next to the name of your app, which is not as obvious.
Facebook just announced at their F8 conference that Apps will be getting overhauled and become more prominent over the next few months, which we're excited about:
developers.facebook.com/blog/post/2015/0...elping-People-Share/
So, check those two areas above for now to see if they're there and test... and soon, hopefully it will be more obvious.
Thanks,
Alex