Why the method getChildView is not called?

Question

I'm developing an Android app and I'm having trouble with expandableList. I found a sample code on the web and I tried to adapt it to my case. I tried to compare my code to the code I found on the web and they looks like very similar, but my code didn't work. When I try to run the app the phone load correctly the view with the expandable list, it shows the group list but when I tap on the row that I want to expand it doesn't shows anything. My code is the follow:

package it.bitmama.interactv2.adapter;

import it.bitmama.interactv2.R;

import java.util.Calendar;
import java.util.List;
import java.util.Map;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.graphics.Typeface;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.widget.BaseExpandableListAdapter;
import android.widget.ImageView;
import android.widget.TextView;

public class ExpandableListAdapter extends BaseExpandableListAdapter {
    private Activity context;
    private List<String> episodes;
    private Map<String, List<String>> episodeData;  

    public ExpandableListAdapter(Activity context, List<String> episodes, Map<String, List<String>>episodeData) {
        this.context = context;
        this.episodes = episodes;
        this.episodeData = episodeData;
    }

    @Override
    public Object getChild(int groupPosition, int childPosition) {
        return episodeData.get(episodes.get(groupPosition)).get(childPosition);
    }

    @Override
    public long getChildId(int groupPosition, int childPosition) {
        return childPosition;
    }

    @Override
    public View getChildView(final int groupPosition, final int childPosition,
            boolean isLastChild, View convertView, ViewGroup parent) {
        final String episode = (String) getChild(groupPosition, childPosition);
        LayoutInflater inflater = context.getLayoutInflater();

        if (convertView == null) {
            convertView = inflater.inflate(R.layout.child_item, null);
        }

        Log.d("das", "" + episode);

        TextView item = (TextView) convertView.findViewById(R.id.episodeTime);

        ImageView add = (ImageView) convertView.findViewById(R.id.addCalendar);
        add.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                Log.d("EVENTI", "Aggiungere al calendario");

                Calendar cal = Calendar.getInstance();
                Intent intent = new Intent(Intent.ACTION_EDIT);
                intent.setType("vnd.android.cursor.item/event");
                intent.putExtra("beginTime", cal.getTimeInMillis());
                intent.putExtra("allDay", false);
                intent.putExtra("endTime", cal.getTimeInMillis() + 60 * 60 * 1000);
                //intent.putExtra("title", title);
                context.startActivity(intent);

            }
        });
        item.setText(episode);
        return convertView;
    }

    @Override
    public int getChildrenCount(int groupPosition) {
        return episodeData.get(episodes.get(groupPosition)).size();
    }

    @Override
    public Object getGroup(int groupPosition) {
        return episodes.get(groupPosition);
    }

    @Override
    public int getGroupCount() {
        return episodes.size();
    }

    @Override
    public long getGroupId(int groupPosition) {
        return groupPosition;
    }

    @Override
    public View getGroupView(int groupPosition, boolean isExpanded,
            View convertView, ViewGroup parent) {
        String episodeName = (String) getGroup(groupPosition);
        if (convertView == null) {
            LayoutInflater infalInflater = (LayoutInflater) context
                    .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = infalInflater.inflate(R.layout.group_item,
                    null);
        }
        TextView item = (TextView) convertView.findViewById(R.id.episodeName);
        item.setTypeface(null, Typeface.BOLD);
        item.setText(episodeName.substring(0, 25));
        return convertView;
    }

    @Override
    public boolean hasStableIds() {
        return true;
    }

    @Override
    public boolean isChildSelectable(int groupPosition, int childPosition) {
        return true;
    }

}

What's wrong? Thank you