Lost credits (Activated by gift card)!

At the same time, I activated 2 gift cards (on 10$ and 20$). After that, I tried to make a purchase, but nothing worked. I started to change the data Apple ID, thinking that the problem is there. At the moment of action, I observed that my credits gone ($ 30). After 6 hours, my $ 10 back . Just change the password, I was able to buy a few applications. Problem: I do not back another $ 20, and where they all gone? I have not received any letters!

Help me please!