Home delivery:
Everything indicates that the parcel has been delivered, but the customer has not received it.
When the Post Office can't find you on the day of delivery. The deliveryman often leaves the parcel with a third party: janitor, neighbor, local shop. First check this point before starting a carrier investigation.
Delivery to a relay point:
It may be that the customer has forgotten to select a relay point or forgotten the address.
It is also possible that the relay point is closed or overwhelmed and that the parcel has been assigned to another relay point.
The customer can find the chosen relay point in the carrier's tracking link and in the emails sent.
If the parcel cannot be found despite all efforts, you should contact the carrier to obtain more information regarding the delivery.
Here is the procedure to follow: https://help.faume.co/en/articles/9504332-open-a-carrier-investigation