If Royal Mail are unable to obtain a signature from somebody at the address then your parcel will be returned to your nearest Royal Mail depot. Royal Mail will leave a card at the delivery address, which will have instructions of re-delivery or collection.
If a FED EX shipment can not be delivered, a card will be left and the courier will attempt re-delivery on the following working day. Following this, if delivery is still unsuccessful a card will be left and the goods returned to the depot, from here a collection or re-delivery can be arranged.
If a DPD - Next day delivery is undeliverable at the address : items may be left with a neighbour or in a safe place unless you specifically say that you're not happy with that arrangement.