Pay for data add on successfully, but still cannot use data

I am in $30 month plan, AT&T prepaid. I bought a $5 for 250 MB data add on, and my bank account already shows I have $6.61 (include tax) transaction with AT&T.

However I still cannot use data, at the beginning I thought it was my phone's problem, but I found I cannot use data both in my unlocked Samsung and AT&T Nokia Go Phone.

Then I log in to my AT&T account, I find the system says I have no data add on! But I have paid, and I even got the message saying my prepaid payment is successful! Why?