callback function problem with sandbox test user (IAP)

I test in-app purchases on ios with sandbox test user and real test device.

Steps:

• It prompted for an ID/Password (sandbox user).

• purchase dialog appear with the text that the payment was successful

And problem is that sometimes callback function is not called. When i click on buy button again (right after closing the successful transaction popup window) it instantly call callback function (without previous steps). When i click on buy button again it start over so it prompted me for an ID/Password (sandbox user). So i must click on buy button again after purchase dialog (with the text that the payment was
successful) is closed. The same problem with consumable and
non-consumable purchases.

But sometimes it works as expected so callback function is called
automaticlly after purchase dialog with positive result is closed. Also
restoring transactions works always as expected.

Is there some problem with sandbox enviroment?