mtn not crediting correct data every month

My name is Nhlanhla Nkuna cell *** Mtn has been crediting with the wrong Data i am supposed to be getting 30 gig every month for what i took with MTN for 24 months and they have been creditng me with 20 and 25 it differs everymonth and the contract i took its for 30 gig and they are debiting me for30 gig ref *** please