setOnItemSelectedListener of Spinner does not call

Try this

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

        //to fill your Spinner
        List<String> spinnerArray = new ArrayList<String>();
        spinnerArray.add("Item 1");
        spinnerArray.add("Item 2");
        spinnerArray.add("Item 3");
        spinnerArray.add("Item 4");
        spinnerArray.add("Item 5");

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                android.R.layout.simple_spinner_item, spinnerArray);
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        Spinner spinner = (Spinner) findViewById(R.id.quantity);
        spinner.setAdapter(adapter);

        spinner.setOnItemSelectedListener(new OnItemSelectedListener() {

            @Override
            public void onItemSelected(AdapterView<?> adapterView, View view,
                    int position, long id) {
                Object item = adapterView.getItemAtPosition(position);
                if (item != null) {
                    Toast.makeText(MainActivity.this, item.toString(),
                            Toast.LENGTH_SHORT).show();
                }
                Toast.makeText(MainActivity.this, "Selected",
                        Toast.LENGTH_SHORT).show();

            }

            @Override
            public void onNothingSelected(AdapterView<?> adapterView) {
                // TODO Auto-generated method stub

            }
        });
    }