I send a lot of packages out, and have used most carriers and couriers and am back to the RM. I occasionally have stuff delayed with them, but overall they get most packages to the right place at the right time. I had some delayed recently, but the snow and covid will have had an effect. What i do find weekly is people not giving the full address, no flat number as an example and the grander the address the more likely this is to happen. So almost weekly i will get 3 the castle or similar, and when i log into the post office gazzateer to send their package i am given a choice of 1/60 3rd floor, the castle. Its often automated, so it will go for 1/3 the castle as a default and if its an ebay purchase it will just not get delivered properly. This are the ones where it says red card dropped off but its simply not put through the letter box, as they dont know which letterbox to out it through , and you need to go to the parcel office to collect it. Another problem is giving their house a name with a plate of eBay and not registering this. Again i see this weekly and its incredibly foolish, as its the same gazateer that the emergency services use. So if you live at appartment 4, the stables, and buy "red Rum" nail it to your door and expect the world just to know, then you are going to have some mail delayed, and the emergency services will be delayed in finding you. The RM are not perfect, but when things dont go well then i find there is almost always a story, a reason why its not gone right. Its often the RM fault but not always.
These are not comments on the OP situation, i can offer nothing to assist in that case, but my own personal experiences are that overall stuff gets to the right place eventually. In the case of the OP there is a situation where the object is marked as delivered when it is not, and i forget what this is. Its returned to the delivery office. From memory its something like once the posty scans it if he then rings the doorbell or knocks, if no one comes to the door then he cannot un scan it, or scans the wrong package then it cannot be unscanned, something like that, Its happened a couple of times to my customers, causes a mass of hassles and when explained its simply something is done in the wrong order, the package is safe but not delivered. The pubic tracking on the website does not show this, but speak to the staff and they can see further details may be able to sort it in seconds. Hopefully it is something like that and it will be quickly sorted at the depot.