No response from MTN - data gets inexplicably depleted within days

For over a year I have been struggling with the fact that my data (at least 15GB) gets used up within a few days of the month. I have contacted 5 stores, logged numerous support calls and finally logged a technical request to analyse my data used to be able to pinpoint the problem. No-one to date has been able to give me any reason as to why this is happening nor does anyone get back to me. Each month I am paying my monthly subscription and have had to switch off the mobile data option on my phone. I am helpless to proceed and to get any resolution.