Docusign checkbox is invisible

I add two checkbox using java SDK. One uses AnchorString relative position, one uses absolute position. Somehow the one use absolute position just be invisible. The codes to create the two tabs:

    Checkbox checkbox = new Checkbox();
    checkbox.setTabLabel("chkLabel");
    checkbox.setPageNumber("1");
    checkbox.setAnchorString("관련");
    checkbox.setAnchorXOffset("-10");
    checkbox.setAnchorYOffset("-10");

    Checkbox checkbox2 = new Checkbox();
    checkbox2.setTabLabel("chkLabel2");
    checkbox2.setPageNumber("1");
    checkbox2.setXPosition("100");
    checkbox2.setYPosition("100");

    List<Checkbox> checkboxTabs = new ArrayList<Checkbox>();
    checkboxTabs.add(checkbox);
    checkboxTabs.add(checkbox2);