My grandparents are Holocaust survivors, they wont talk about it at all, is this reasonable?

My family is Jewish, from German/Polish Jewish background. My grandmother and grandfather on my mother's side survived the concentration camps...They still have the numbers tattooed on their arms from Auschwitz, they lost all of their family members, but they have never talked about it to me or my siblings at all, hardly mentioned anything and won't ever go into any detail about any of their experiences. Although they're very loving and close to me and always have been. I've tried to ask them many times but they just avoid it. How can I get them to tell me their story? I feel it should be known and I would like to have a record of it. Or, Should I just not ask them again?