Bad service!!!

I have been with mtn since 2011. Last year August i took a contract sim of 80gb monthly, however mtn have NEVER given me that 80gb if its not 53 gb its 60gb then during the month i will run out data! I have been going to their stores but they cant resolve ANYTHING. I dont understand why do they market something but dont deliver! This is bad im so disappointed i need to cancel that contract ASAP because WOW