Did you switch to the Pre-Paid Ultimate Plus plan? They've had the PrePaid Ultimate plan for a while but you'd need to change across to the new one to get the offer.
If you did change across, they actually split it into 7GB of standard data and 7GB of bonus data. The bonus data can take a few days to appear on the account. Given you said you only got 6GB though, I suspect you're on the old plan