getPendingPurchasesIOS method is not returning the pending transactions.

[Karthik-V26]

Hi,
I would like to solve a issue, where if the transaction successful on the App level and did not update in the backend(issues like server down, internet etc...) I trigger the finishTransaction() only after the transaction gets updated in backend.
But when I reopen the app, sometimes i am getting update event from purchaseUpdatedListener() but sometimes not.
So I tried with the getPendingPurchasesIOS, this is also not working properly.
Any other ways to check this? or Am i missing something?
Is there any things which needs to consider while using these methods ?