IIRC 10 working days before you can put a claim in but deffo contact RM customer services and complain about the situation and especially the manager being too busy to see you and failing to call back when promised.The delivery office gets an email and the manager has to respond to it.
Non delivered items normally get returned to office on completion of duty around 1430 ish but might not get scanned back into the system until next day depending upon how busy the guy in the office is.
Don’t know why it shows up on track n trace for you but they can’t find it, maybe he input the number wrong, although they can check by postcode or surname. I know it doesn’t help you but things go missing quite often either accidentally left in a mail bag (postie doesn’t necessarily use the same bag every day) or in back of the van.Packets are not so bad but letters easily get mixed in with a collection so end up going around the system again.
Good luck and top tip if it’s ever something important use Special delivery.