Cannot reroll heroes from my 2nd hero challenge

After having done my first, it suprised me to see I had no option to reroll my 2nd hero challenge. I've seen the previous posts about clicking on the "Give new heroes" button making it so you cannot reroll anymore, but that's not the case here, didn't do that.

http://i.imgur.com/NLzAoOT.jpg

Am I missing something here?