Nested Viewpager Fragment not getting initialized

Question

I have 3 fragments part of a navigation drawer, one of which contains a viewpager. When I first load the app, the viewpager loads all the fragments. However, when I switch to another fragment using the navi-drawer and I switch back, one of the viewpager fragments is not loading. I have tried the code here and here but I'm not able to come up with a solution.

In my pageradapter code, in the switch case for different fragments; I have put a log for the position, and position 1 does not seem to be coming after the initial load of the app.

My question is, why is it that position 1 never gets called? Can someone explain it to me?

Here's my Pageradapter code:

 class PagerAdapter extends FragmentStatePagerAdapter {

static HeartRate_Fragment heartRate_fragment;
static Calorie_Fragment calorie_fragment;
static  Steps_Fragment steps_fragment;

int num;
public PagerAdapter(Context context, FragmentManager fm) {
    super(fm);

}
@Override
public Fragment getItem(int position) {
    //add new fragment here for homepage viewpager
    Log.d("position",position+"");
    switch (position) {

        case 0:
           if(calorie_fragment == null)
            calorie_fragment = new Calorie_Fragment();
           Log.d("flip",0+"");
            return calorie_fragment;

        case 1:
            if(heartRate_fragment == null)
                heartRate_fragment = new HeartRate_Fragment();
            Log.d("flip",1+"");
            return heartRate_fragment;


        case 2:
            if(steps_fragment == null)
            steps_fragment = new Steps_Fragment();
            Log.d("flip",2+"");
            return steps_fragment;


        default:
            return null;

    }

}

@Override
public int getCount() {
    return num;
}

public HeartRate_Fragment getHRFragment()
{
    return heartRate_fragment;
}
public Calorie_Fragment getCalorie_fragment()
{
    return calorie_fragment;
}
public  Steps_Fragment getSteps_fragment()
{
    return steps_fragment;
}

Answer 1

class PagerAdapter extends FragmentStatePagerAdapter {

static HeartRate_Fragment heartRate_fragment;
static Calorie_Fragment calorie_fragment;
static  Steps_Fragment steps_fragment;

int num;
public PagerAdapter(Context context, FragmentManager fm) {
super(fm);

}
@Override
public Fragment getItem(int position) {
  //add new fragment here for homepage viewpager
Log.d("position",position+"");
switch (position) {

    case 0:

        if(calorie_fragment == null){
        calorie_fragment = new Calorie_Fragment();
        return calorie_fragment;
        }else {
         return getCalorie_fragment(); }         

    case 1:
        if(heartRate_fragment == null){
            heartRate_fragment = new HeartRate_Fragment();
            return heartRate_fragment;}
         else{ return getHRFragment();}

    case 2:
        if(steps_fragment == null){
        steps_fragment = new Steps_Fragment();
        return steps_fragment;
        }
        else{ return getSteps_fragment();} 

    default:
        return null;

}

}

@Override
public int getCount() {
return 3;
}

 public HeartRate_Fragment getHRFragment()
  {
  return heartRate_fragment;
  }
  public Calorie_Fragment getCalorie_fragment()
  {
  return calorie_fragment;
  }
  public  Steps_Fragment getSteps_fragment()
  {
  return steps_fragment;
  }

try this make sure you are using getChildFragmentManager();