Yes, if carrier does not scan on delivery, this can cause issues.
However see latest update, you will no longer have to wait 14 days in some circumstances.
For orders shipped through Simple Delivery, you will no longer need to wait 14 days to access your funds if delivery confirmation is not received. If an item is dispatched within 3 calendar days, we’ve reduced the payout time so funds will now be available 8 calendar days from the order date or 2 calendar days after we receive delivery confirmation, whichever comes first. This change will be live in the next week.