In app purchase subscription renewal not working as expected

Question

I implemented in-app purchase in my application.
First time it asked for itunes authentication and purchase confirmation.
I store the success receipt in NSUserDefaults.
After the expiry time (I am testing in Sandbox, so 5 minutes), when I take application, I verify the receipt. Sometimes it returns the status 21006 and sometimes 0. I could not understand why this happens.
Will the apple renew automatically the subscription. If YES, then why some times it returns the status 21006. Should I do anything more for the subscription feature.
Please help me. I am really stuck with this.

Answer 1

I had this similar experience. I'm not sure if you're aware but not only are durations reduced in the sandbox but renewals are limited to 5 times (I think). So if you verify 25 mins after your initial purchase (for example) the subscription should be expired. I found the sandbox to be finicky and not entirely consistent.