User Profile

My app is having an issue with only some users not being able to access Premium features.  Apple will indicate that the payment/subscription was successful, but not allow these users to access to the Premium features.  And if those users try to purchase/subscribe again, a popup will indicate that he/she is already subscribed to this. But again, they can't access the premium features. We can't replicate this internally because it only seems to be an issue with a few users. We can't quite figure out what the issue is and why it's only occurring for some users. Has anyone experienced this?