paymentQueue FinishTransaction never gets called after in app purchase

I have implemented this StoreObserver and I have registered it in AppDelegate but my paymentQueue func never gets called whenever there is a successful purchase. The reason I need it to be called is firstly to finish the transaction but to also send the device receipt to the server. Currently, my receipt is always null.

The purchases themselves work OK, both in XCode and in Sandbox.

I can't figure out what I'm doing wrong in terms of the setup of the storeobserver. Any help will be greatly appreciated.

AppDelegate.swift

let iapObserver = StoreObserver.shared

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]? = nil) -> Bool {
    
    
    UIApplication.shared.registerForRemoteNotifications()
    UNUserNotificationCenter.current().delegate = self
    SKPaymentQueue.default().add(iapObserver)
    return true
}

StoreObserver.swift

  class StoreObserver: NSObject, SKPaymentTransactionObserver {
       static var shared = StoreObserver()
    
        func paymentQueue(_ queue: SKPaymentQueue, updatedTransactions transactions: [SKPaymentTransaction]) {
            
            for transaction in transactions {
                switch (transaction.transactionState) {
                case .purchased:
                    SKPaymentQueue.default().finishTransaction(transaction)
                    
//print the receipt but it is always nil. (I will be saving this on the backend for validation)
                    if let url = Bundle.main.appStoreReceiptURL,
                       let data = try? Data(contentsOf: url) {
                        var receiptDataString = data.base64EncodedString()
                        print(receiptDataString)
                    }
                    
                    break
                case .failed:
                    SKPaymentQueue.default().finishTransaction(transaction)
                    break
                case .restored:
                    SKPaymentQueue.default().finishTransaction(transaction)
                    break
                case .deferred, .purchasing:
                    break
                default:
                    break
                }
            }
        }
            
            func paymentQueue(_ queue: SKPaymentQueue, removedTransactions transactions: [SKPaymentTransaction]) {
               //
            }
            
        func paymentQueueRestoreCompletedTransactionsFinished(_ queue: SKPaymentQueue) {
           //
        }
            
        func paymentQueue(_ queue: SKPaymentQueue, restoreCompletedTransactionsFailedWithError error: Error) {
           //
        }
            
            func paymentQueue(_ queue: SKPaymentQueue, updatedDownloads downloads: [SKDownload]) { 
               //
            }
         
        
    }