I merged a section of my family tree with others and the new information "lost" a generation. How can I add the missing generation back in?

After taking on information from another tree, I am missing a generation in my tree, and can't get it corrected,  I tried adding the missing child but the person does not show up in the tree, and this can't be linked to the correct offspring.  I tried simply the parents information over the grandparents, and then adding the grandparents back in in the correct generation, but this simply caused the first names not to show up. The edit relationships box shows only spouses, so does not allow changes to any other relations.