Bad long life milk!

We purchased Kelloggs brand of long life milk from a particular supermarket and upon opening the 6 pack we found 2 1 litres that had been spoiled, they were not expired and they were stored correctly. I understand these things can happen however after complaining directly to Kelloggs they tried to pacify us with a voucher which we accepted however we never received it and now they don't even respond to our emails anymore. With the cost of living nowadays can anyone afford to be discarding 2lts of milk. Completely unacceptable!