Invalid voucher provided after completion of 4-week challenge

I got an email from Discovery on the 21st of April regarding a 4-week challenge which I successfully completed (challenge was to conduct a full Vitality Health Care by the 19th of May) and the prize was a R250 Dischem voucher. I did not get an SMS which they promised to send. I contacted Discovery Vitality on Twitter and was subsequently provided with another voucher. I then proceeded to Dischem, picked out my items and getting to the till, was told the voucher is invalid. I have been given the run around for the past 2 months and am literally fed up. Why would you offer a **** prize for a challenge when you didn't intend on meting out a prize at all. Was it all a trick just to force people to go through the challenge, why lie?