Pretty weird situation. All you can do is ask, but they may not be too forthcoming with information. I've got a slightly different situation: it took me a while to notice at first, but I stopped getting mail on Saturdays in the fall of 1996. I mean, I didn't always get mail on Saturdays before that, but it happened occasionally. Since then, not once, at any address in all those years. Monday to Friday delivery is as normal. I'm mystified.
"Tragedy is when I cut my finger. Comedy is when you walk into an open sewer and die." --Mel Brooks