Less data more money

I have taken a WiFi router from telkom. I am supposed to pay R139 a month for 10gigs. For some reason I have been paying R169 a month and every month I receive a gig less. Today they loaded 7gig on my router but the full amount has been debited from my account. I always thought I would never encounter such problems with Telkom. I suppose by the end of my contract I'll be receiving 100Mb. I would like for Telkom to fix my problem.