Data not received

I have a 500MB data contract with telkom mobile, they claim that they gave me 500MB data on the 1st of December 2015, my tablet is mostly off and I rarely use it for internet. today when i check I found that my data is used up, when I phone them they say they did give me data and I used it for internet. what makes me believe that they did not send me any data this month is that whenever I am left with 99MB I receive an sms alerting me but there's no sms that I received this month. I want my data.