Buying airtime and it keeps disappearing

On more than 2 occasions I have purchased airtime from my Nedbank app, the airtime will load and with in a few hours before Iv gotten to use it or to transfer it into data, it simply just disappears. They are only small amounts (R10 - R20) but it's now adding up!! I'm definitely not using it because I have uncapped wifi at home. I leave my house, want to transfer the airtime into data and then I realise it has disappeared again. Iv been with Cell C for nearly 8 years now and I'm getting cheesed off