There are many reasons why this could be the case, we would recommend checking your courier tracking link.
LEFT IN A SAFE PLACE
Authority to Leave means the courier will not require a signature for delivery, and will leave the order in a safe place. We recommend carefully checking your property for the order.
COLLECTION FROM POST OFFICE
Where a safe place to leave the order could not be identified, the order may be taken to a Post Office nearby instead.
To confirm that has occurred, you can review your order using this more detailed tracking link: https://www.dhl.com.