Whenever I load data, my airtime is used instead of data

<p>I just purchased R17 airtime + R50 airtime , I had a remainder of R8 , in total I then had R75 airtime plus I had received 150mb megadata with a balance of 2mb = 152mb balance R75.17 , my 2mb data bundle depleted and cell c took R10 airtime from my account instead of using the 150 mb , I am left with R66 airtime & 149.56mb , this is disgusting </p>