...and a shaky mailing function in the process.
The challenge is communicated by e-mail. I received my last one on 11-10-2012. Since I responded frequently and there was an active exchange with three players, I can't believe that none of them and nobody else has challenged me since that time.
I rather suspect that there are mails missing and that this is part of the olde "missing mails" subject.
Unfortunately, even Nation's escalated customer service hasn't given satisfying answers to that subject, one mail query waiting for an answer for a week.