Fragment is not shown on Activity

In my App,One Activity has to include three fragments, one of them always remains on Activity but the other two have to change places. But fragments are not shown on an activity.

I have read somewhere here that I should change LinearLayout to FragmentLayout but it did not work.

public class MainActivity extends ActionBarActivity {

private final Fragment_1to6 f1to6 = new Fragment_1to6();
private final Fragment_7to12 f7to12 = new Fragment_7to12();

private FragmentManager mFragmentManager;
private FrameLayout mButtonsLayout, mLayout1to6;
private FrameLayout mLayout7to12;

private static final int MATCH_PARENT = LinearLayout.LayoutParams.MATCH_PARENT;
private static final int WRAP_CONTENT = LinearLayout.LayoutParams.WRAP_CONTENT;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    mButtonsLayout = (FrameLayout) findViewById(R.id.fragment_buttons);
    mLayout1to6 = (FrameLayout) findViewById(R.id.fragment_1to6);
    mLayout7to12 = (FrameLayout) findViewById(R.id.fragment_7to12);

    mFragmentManager = getFragmentManager();
    final FragmentTransaction ft = mFragmentManager.beginTransaction();

    ft.add(R.id.fragment_buttons, new buttonsFragment());
    ft.add(R.id.fragment_1to6, f1to6);

    ft.addToBackStack("fragment_buttons");
    ft.addToBackStack("fragment_1to6");

    ft.commit();

    mFragmentManager
            .addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener() {
                public void onBackStackChanged() {
                    setLayout();
                }
            });


    mFragmentManager.executePendingTransactions();


}

private void setLayout() {

    // Determine whether the Fragment has been added
    if (!f7to12.isAdded()) {

        mButtonsLayout.setLayoutParams(new LinearLayout.LayoutParams(WRAP_CONTENT,
                MATCH_PARENT, 1f));
        mLayout1to6.setLayoutParams(new LinearLayout.LayoutParams(WRAP_CONTENT,
                MATCH_PARENT, 6f));
    } else {

        mButtonsLayout.setLayoutParams(new LinearLayout.LayoutParams(WRAP_CONTENT,
                MATCH_PARENT, 1f));
        // Make the TitleLayout take 6/13 of the layout's width
        mLayout1to6.setLayoutParams(new LinearLayout.LayoutParams(WRAP_CONTENT,
                MATCH_PARENT, 6f));

        // Make the QuoteLayout take 6/13's of the layout's width
        mLayout7to12.setLayoutParams(new LinearLayout.LayoutParams(WRAP_CONTENT,
                MATCH_PARENT, 6f));
    }


}

how can i solve that ?