Search for Andreas Olsen

Born in Nannestad, Akershus, Norway Sept. 18 1862 and his wife Anne Marie born Andreasdatter or Anderdatter, born in Gran, Hadeland, Oppland, Norway, Oct. 9 1867. We can find several listings in Akershus, several places they lived and their children, some of them, incl. my grandfather Hans Martin Olsen (Andreassen) born Aug. 8 1906. But we can't find more information about their ancestors, their parents and other relatives. Does anyone have a tip on how to track them further back in time? Have a good weekend!