Changing from Flexible to Fixed Interest

So I put through a request to Nedbank to change my interest rate on my Home Loan from flexible to fixed. Today I received an email saying it was declined as a result of affordability. What I don't get it is, when they increase the installment they do not look at one's affordability . I'll probably be paying more by the end of the year, than the proposed amount they had said I would pay if the interest was fixed. The agent tells me that I should contact the collection dept if I cannot afford it, which will increase the term and the interest. I'm not saying I don't want to pay, all I want if for the interest to be changed and I can afford that installment every month. I've never been so disappointed. Definetly to switch my HL.