Very poor service from MTN re lost data for no reason

I have a 4GBs bundle per month with MTN which corresponds to 2Gbs daily usage and 2 GBs night express. This contract has now lasted for about 2 years and on the 12th of every month at midnight whatever data left from the month before expires and the 4GBs are renewed. Nov. 11th when I last checked my data I had 710MBs still left. This was at around 21h30 when I last checked it. Then I went to sleep. The following morning around 7h20 I checked my data again and my to my surprise, I only had 2.69GBs left of my just renewed data bundle. Since I didn't use my phone all night because I was sleeping, where did 1.31GBs go to? I contacted MTN straight away but all agents are beating around the bush or not even replying to me when I ask what happened to this 1.31GBs. Clearly a mistake from their side which they don't want to admit. I need an answer, an apology and the 1.31GBs credited to my bundle since I didn't use it. It's now been more than a week since I first contacted them.