To access your booking you need to be logged into the account with the same email address as your booking confirmation email. If you login to a different account, you won't be able to see your booking. If you used social login to autofill your details on Apple ID, you may have entered a different email address to make the booking. To check this, go to "Edit Details" in your app.
If you have two Hostelworld accounts but only want one, please email us with which email you'd like to keep. You will need to send us this via both email addresses so we can merge the accounts.