Data vanishing,how possible is that?with MTN it is

I have a problem with mtn, everytime i receive data on the 21 of each month it vanishes and I always ensure that data network is always off and I once took it to mtn branch and they turned off everything they suspected that it might be consuming data. With my data network being turned off I expect to have the same balance I had today but that is not the case, data keeps on vanishing and no one wants to account. I am not going to renew or upgrade my contract with you as you are not trustworthy and I am going to try other networks but with you I am not happy at all