Short Payment on Pre-Approved Authorization provided.

I fail to understand how a medical aid scheme, who are receiving their money on a monthly basis (deducted from your salary) is not able to cover your hospital bill. I have never in my life been hospitalized or claimed from my Medical aid for operations, and now for the first time Sizwe is failing to settle my hospital bill submitted for payment. After pre-authorization has been provided to the Service provider they only paid 1/3 of the bill expecting me to settle double the amount they paid. How the payment process works is beyond me. I tried to get ahold of them for the past week with no success. I need clarification/explanation form them on why they short paid my bill. If this is not resolved I'm definitely moving away from this Scheme. This is a terrible experience, where I am afraid to get sick or go to a doctor fearing that they will not be paid for the service rendered. I need answers as the service form Sizwe is poor.