Poor services at Mercedes-Benzes and unfair treatment to customers

I am not happy about the services from Mercedes-Benzes of South Africa, they are refusing to fulfil their agreement in terms of performing the services as per my contract with them. I extended my service plan with them and I informed the consultant to replace the brake pads on my last schedule date, and it was not done, but now they are demanding me to pay for the brake pads. I've been a customer for Mercedes- Benzes for so many years, but now this is how they treat me now.