Data Expired One hour after purchase

I purchased airtime from Cell C via internet banking to the value of R150.00 on the 01/08/2015 at 21:03:27. I immediately converted the airtime to 1GB of data through the USB modem app on my pc and received notice confirming that I had received the 1GB data and 400MB free data by sms to the application message facility. An hour or perhaps slightly longer later I received another sms to say that my data is expired after I had only minimal email and internet usage. I notified Cell C after I was able to get my purchase history from the MyCell C website on the 4th August and they have not been able to give a satisfactory response or resolve the issue since then except to say that my current query is still receiving urgent attention. I find it difficult to believe that they have not been able to verify that the airtime was purchased and converted to data and check the record of usage in ten days. I am unwilling to use their service or buy more airtime until they can resolve the issue.