Royal Mail
If you have a tracking number check the status of your order in this link:
https://www.royalmail.com/track-your-item
Royal Mail 48 and 24 tracked is traceable all the way through its journey as is special delivery.
From the end of September 2022, if there is no recipient in to accept your parcel(s), we may choose to deliver them to a Safe place location at the address, if our postman or postwoman deems that location to be suitable (currently we only leave parcels in a Safe place if the recipient has asked us to directly). In that case, we will also leave a "Something For You" card explaining where the parcel(s) is located. If we are unable to find a suitable safe place and cannot deliver to a neighbour, we will return your parcel(s) to our local Royal Mail Customer Service Point and leave a "Something For You" card at the address.
Have they left it somewhere close by or with a neighbour?
Neighbours wont always tell you they have your parcel as they assume you will know and are probably too busy to come and let you know.
DPD
DPD should text you before 11am on the day of delivery to let you know within which hour they will deliver.
Have they left a card if you were not in when they attempted delivery? Follow instructions on the card to re-arrange delivery.
Have they left it with a neighbour?
Neighbours wont always tell you they have your parcel as they assume you will know and are probably too busy to come and let you know.