Airtime that get lesser and lesser every month

I have a VIrgin Mobile Account that i took out for my daughter . for the past seven months the airtime is getting lesser. From R200 to R39 a month. This is rediculous and pure ****************. The airtime is getting lesser but the payment on a monthly basis stay the same. This is not the first complaint i have with Virgin Mobile for the same thing!!!!!. i need all the airtime to be refunded back to the account . and believe me this is the last i am going to have a contract with VIrgin Mobile. My wife have a Vodacom contract and she does not have these problems. I have Cell C contrat for the past 15 years and no trouble. Come on Virgin Mobile , maybe you can learn from them