Night express data not working

I am on contract and I get 500mb to use during the day and 500mb to use in the night time between 1am and 5am. I stay up to upgrade my phone apps during this time but i have noticed that my night express is not working as it should. when i upgrade, my day time data is charged and once that is finished, it then uses my airtime. I checked my balance and had no day time data left but night express was still on 500mb. I called twice with no resolve. its been over a week now and my nigh time is expiring on the 21st and i cant use it. MTN cant even help me. I spoke to Msizi on 27/09, he said it will be fixed in 24 hours but it wasnt. I then spoke to Thabisile on 3/10 . She said it will be fixed on 10/10 but still... no fix. I called again on 13/10 and Nokuthula said on 17/10 it will be fine. still nothing. I'm starting to think MTN cant do their work. please fix this. I pay every month and this is not what i pay for. I want to use my night express before it expires.