Unable to reach clients?

How often do you get clients that just sort of go missing? Not literally… Or maybe literally. I do workman’s comp at a pretty sizable firm, and while I currently only work for one attorney, they have a lot of clients. There have been 3 clients in the last 2-4 weeks that have not responded to multiple attempts to contact them, and certified mail is coming back undeliverable. That’s not a lot considering how many they have, but since I’m pretty new and have only just started interacting with clients, it feels strange to have multiple people that are MIA. Anyone else had this happen to them? Especially in PI/Workers’ comp fields?