Verizon data on my ipad mini won't work?

During the month of my data, I have to add more once or twice a month. Usually, it adds quickly without any problems, but recently, it has been saying system error. I called Support, but no help. The lady who assisted me was very rude and confusing, and at one point laughed because I didn't know something about my data. She said she'd fix it, but never did? This was a week ago. My debit card number is correct, and there isn't a problem with it. Help?