De*****ed

I am being de*****ed by Virgin Active they giving me discounts on my installments then we agree over the phone to the discount then when I pay they keep saying my discount somehow did not work or is not on the system but before I pay it apparently shows I qualify for the discount but the reason why they gave me a discount in the 1st place is because they caught on nonsense in the call centre so how can this issue be resolved or are they just going to get away with it