Wrong Voucher Sent

On the 4th of June I won a R200 Nandos voucher competition from Huawei and sent them all my details as requested, they didn't send the voucher and I followed up many times until they send it via twitter, I went to Nandos to buy only to find that they sent me a R100 voucher instead of the R200 I won. I sent them several messages and they don't reply.