Pathetic service MTN

I have taken the black Friday deal from 2018 on a SIM only and paying 358 and was getting 71 gigs of data every month aside from the free Min and sms, however from January this year I stopped getting 30 gigs which still says on sms that I am subscribed to recurring 30 gigs .I am still paying 358 a month and only receive 40 gigs.i Had received a call last week saying I have a credit but nobody contacted me and what is the credit for cos I have received no money back neither do I have extra data.From March I am in constant calls and emails but nobody from MTN nlknows nothing.pathetic service.still awaiting one manager from any department not MTN to call me as I left numerous messages.