I actually found a way around it that worked for me at least. It was by accident, but it worked.
Try this.
1. Get to the screen where you would collect the reward that you are unable to collect.
2. Exit the app without closing it down. So just hit the center button or whatever.
3. Wait a sec and then reenter the app through the one screen where you see all the apps you have used, but haven't closed down yet. (This screen you get to by double tapping the home button or swiping up, depending on you device).
It should be collectable after that.
Just let me know if this works for you like it did for me. Sorry if this is confusing. It's actually really easy.