Vizio auto search skips channels

I have Vizio 37XL. When I recently auto searched it found 24 analog 
and 47 digital channels with my OTA antenna. However, it did ignore 
NBC 4-1 and 4-2 which I was receiving perfectly well till today with 
the previous auto search performed a week ago. Is there a way to 
add channels? The only choice  on the menu is to remove 
channels. I do not see any way to select a digital channel which was 
not discovered during auto search. 

Cass