Spinner with sub-spinner not working as expected

I have two spinners, such that my second spinner changes options that it can offer according to the item selected in the first spinner. Easy?

Example: If I select 'a' in main spinner, the sub spinner should show 'a1' as option. If I select 'b' in main spinner, the sub spinner should show 'b1','b2' as options. If I select 'c' in main spinner, the sub spinner should show 'c1','c2','c3' as options.

I use a library called SearchableSpinner but that does not matter as it works just like the Android spinner.

public class PostComplaint extends AppCompatActivity {
    String[] problems_main = {"a","b","c"};
    String[][] problems_sub = {{"a1"},{"b1","b2"},{"c1","c2","c3"}};

    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_post_complaint);

    spinner_main = (SearchableSpinner)findViewById(R.id.spinner_main);
    spinner_sub = (SearchableSpinner) findViewById(R.id.spinner_sub);

    ArrayAdapter<String> spinnerAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_item, problems_main);
    spinnerAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    spinner_main.setAdapter(spinnerAdapter);
    spinnerAdapter.notifyDataSetChanged();

    spinner_main.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
            setSubSpinner(position);
        }
        @Override
        public void onNothingSelected(AdapterView<?> parent) {
            Toast.makeText(PostComplaint.this, "Nothing selected", Toast.LENGTH_SHORT).show();
        }
    });
}

void setSubSpinner(int i){
    String[] myArray = problems_sub[i]; //Note: problems_sub is a two dimensional array
    ArrayAdapter<String> spinnerAdapter_sub = new ArrayAdapter<String>(this, android.R.layout.simple_spinner_item, myArray);
    spinnerAdapter_sub.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    spinner_sub.setAdapter(spinnerAdapter_sub);
    spinnerAdapter_sub.notifyDataSetChanged();
}

Problem: Whichever item I click on the main spinner for the first time, according to that the sub spinner is selected. Then if I change the main spinner, the sub spinner is not changing.

The question is open for suggestions. Comment if it is not understandable.