Nginx stops working

I am using Nginx as a reverse web proxy. I host several sites with 1 public IP address. Almost every day when I go to one of the websites I am greeted with a default Apache webpage instead of the hosted site. I reboot the Nginx server and it all starts working again. Why is it doing this?????

BTW - All sites, router, and Nginx are running on a Hyper-V server. That shouldn't have anything to do with it but....