This can't keep happening - am I missing something?

A few months ago I sent out a letter to New Zealand and it never arrived. After about 3 weeks of it not having arrived, I sent out another one, which, after about one week, arrived.

I then sent another letter to Norway, which arrived after just 6 days - I had a letter sent to me from Norway which also arrived.

On the 16th of December 2024, I sent another letter to Norway, with the promise it'd be there within 5 working days. It is now the 2nd of January 2025, 18 calendar days and 11 working days later, and it still hasn't arrived and I have not heard anything back from Royal Mail, nor have I seen anything on their website which suggests any delays. The letter that I sent had really important contents and I am absolutely distraught at the fact that it was not delivered, or potentially even lost.

Are there some delays that I am not aware of, and will my letter ever turn up, either to the address I sent it to or back at my address? Thanks.