AT&T owes me $500 deposit !!!

I am an international study so I had to put down a $500 deposit when I opened my line in the family plan, which was over a year ago.
There are a total of 7 people in our family plan and we all paid the deposit on the same day and in the same store, but the other 6 members all got back their deposit and only I didn't, they all paid with credit card and I paid with cash.
Now I have been contacting AT&T for too many times and each time they told me there was only record of 6 payments and my payment was not on the record. But I have my receipt ! And I also went to the store where I placed the deposit and they pulled out all of our receipts of payment and they also contacted AT&T for me while I was in the store for the whole afternoon and they provided everything AT&T asked for but still no solution ! Now Everytime I call the representative would create a new case number for me and told me that they would contact me but the problem is still not solved and I still don't have my money back! I don't know what the problem is with the system but I have a receipt of my payment !!!
Please help!