Where is my mail?

I have a post box in Kensington. Whenever I find mail in the box, there is ALWAYS something for the box next to mine. Today I had 5 pieces of mail, 4 of which were for the neighbouring box. I always re-post the wrong mail, but as much of the mail I expect to receive never arrives, I can only assume that my mail is landing in someone else's box, and then in the trash. I have asked inside the post office on NUMEROUS occasions, and the situation continues. It is a simple mistake, but one that should to be happening!