iOS auto-renewable subscription won't renew

I'm implementing auto-renewable yearly subscriptions to one our apps. I registered to the SKTransactionQueue on App start:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions 
{
    // Attach an observer to the payment queue
   [[SKPaymentQueue defaultQueue] addTransactionObserver:self];
   return YES; 
}

The purchase of the subscription works fine. However, the subscription won't renew.

-(void)paymentQueue:(SKPaymentQueue *)queue updatedTransactions:(NSArray *)transactions

This function is never called when I launch this App after the 60min renewal time in the sandbox.

What are possible configuration issues, that the app won't get renewal transaction.