Only one checkbox appears in the CheckedTextView

I'm trying to use multiple checkboxes through a listView. But only the first checkbox connected to the first element of the array appears to me. The Class:

        ArrayList<String> selectedItems = new ArrayList<>();
        ListView lvCheckboxList;

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

        lvCheckboxList = (ListView) findViewById(R.id.lvCheckboxList);
        lvCheckboxList.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);

                    String[]  stringa = {"a","b","c"};
                    ArrayAdapter<String> adapterName = new ArrayAdapter<String>(SingleSiteActivity.this, R.layout.check_list, R.id.checkboxList, stringa);
                    lvCheckboxList.setAdapter(adapterName);

                    lvCheckboxList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
                        @Override
                        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                            String selectedItem = ((TextView)view).getText().toString();
                            if (selectedItems.contains(selectedItem)){
                                selectedItems.remove(selectedItem);
                            } else {
                                selectedItems.add(selectedItem);
                            }
                        }
                    });

The xml file where there is the CheckedTextView consists of these few lines:

    <?xml version="1.0" encoding="utf-8"?>
        <CheckedTextView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/checkboxList"
        android:layout_width="match_parent"
        android:layout_height="400dp"
        android:gravity="center_vertical"
        android:checkMark="?android:attr/listChoiceIndicatorMultiple"
        android:padding="5dp"/>

While the main xml file containing the ListView is this:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.XXXXXXXX">







                <ListView
                    android:id="@+id/lvCheckboxList"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content">

                </ListView>



                <Button
                    android:id="@+id/bSelection"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    android:text="SELEZIONA"/>


</android.support.constraint.ConstraintLayout>