Royal Mail tracking is only showing "Sender despatching item" - Why haven't you passed on the parcel yet?!
Sadly, Royal Mail are using a sneaky trick that makes it look like we are at fault. The "sender despatching item" is actually the exact time stamp they collect the parcels from the sender (it gets updated when the postman scans the paper manifest). If Royal Mail have not received a parcel, the tracking result would only state "We are awaiting more information, please check back later". The next scan happens when they will process the mail within their mail centre.
When Royal Mail cannot handle the shipping volume, they will just park all the cages with parcels within the mail centre and not touch them until they can start feeding them into their system. Moving forward, they will prioritise newly received parcels over already late ones and only start processing older parcels that are late once they have free capacity. The reason behind this behaviour is, that they are punished by the regulator when their late delivery rate surpasses a certain level. Since the late parcels are already overdue, they will make sure that new parcels are handled with priority to avoid a knock-on effect.
We are seeing this regularly, especially around holidays or big sales events like Black Friday. This mainly affects shipments handed over after weekends or bank holidays, easing within a day or two thereafter. This always varies by despatching mail centre and will be different from region to region and the time of parcel collections. Generally, this impacts economy services like Tracked 48 more than faster services like Tracked 24 or Special Delivery.