Data bundles

I bought vodacom 20 gb day and 20 gb night contract but every month they are topping up my account with 18 gb, what happened to the remaining 2gb/4gb? Data is expensive and it really is disappointing of them to just offer 4gb lesser than they promissed. When trying to reach their contract customer care, the consultants are always rude and don't even resolve matters. Secondly, I am busy browsing after 12pm but the day data bundles are used rather than the night owl.