Whilst our carrier partners mostly deliver a 95<98% first time success rate, there are the occasional parcels (2<5 in 100) that can sometimes take a little longer, due to all of variables involved in getting your parcel to where it needs to get to.
Firstly, your collection driver will have many collection stops, following many delivery stops that morning, before getting back to his / her depot on time, to meet the last trailer.
This trailer will then need to get to the central hub in time for your parcel to be unloaded and then loaded to the relevant conveyor, passing through many 'hands' for your parcels onward journey.
This then needs to be loaded to the relevant trailer (assuming it has sorted & therefore loaded to the right trailer, otherwise this was cause a 24hr delay) and then line hauled to the delivery depot, in time for the delivery sort.
Once unloaded at the local delivery depot, your parcel will need to be sorted into the delivery drivers routes, often with 100+ other parcels / stops, for the courier to deliver, assuming no hold ups or issues on route.
Then, with all things being well, i.e. no punctures, vehicle breakdowns, motorway closures / severe traffic, adverse weather, security alerts, staff shortages / sickness etc, etc, your parcel 'should' arrive the next day, and even better, within the time slot estimated by the carrier, but as you'll hopefully appreciate, there can be no guarantees.