Mtn contracts

I have a big credit limit at Mtn. There is still a lot of credit available. They keep on declining my application for contracts. When I phone them they tell me that I have a bad credit score. If I have a bad credit score why did they give me contracts in the first place. My account is paid every month before the date for payment. I changed from Vodacom to Mtn. This is not on. I want my contracts to be approved.