onReceiver of BroadcastReceiver not called, AlarmManager

I am building a cab booking app, I need current location of the cab every 20 seconds.

I have defined a AlarmManager and need it to repeat itself every 20 seconds. But its not repeating itself regularly. Instead it repeated itself after 233 seconds, and just once. What am I doing wrong here ?

My HomeScreen has a inner class OnAlarmReceiver, in the onCreate of my HomeScreen I am calling AlarmManager

    AlarmManager mgr = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
    Intent i = new Intent(this, OnAlarmReceiver.class);
    i.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    PendingIntent pi = PendingIntent.getBroadcast(this, 0, i, PendingIntent.FLAG_CANCEL_CURRENT);
    Calendar cal = Calendar.getInstance();
    cal.add(Calendar.SECOND, 20);
    mgr.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP,
            cal.getTimeInMillis(), God.UPDATE_PENDING_INTERVAL, pi);

Inner class in HomeScreen

public class OnAlarmReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        // PullPendingRequests.acquireStaticLock(context);
        Toast.makeText(context, "Don't panik but your time is up!!!!.", Toast.LENGTH_LONG)
                .show();
        Log.d("Taxeeta:PullPendingRequets", "CallService Location");
        context.startService(new Intent(context, PullPendingRequests.class));
    }
}

My AndroidManifest file has

    <service
        android:name="com.taxeeta.support.PullPendingRequests"
        android:enabled="true"
        android:label="@string/app_name"
        android:screenOrientation="portrait"
        android:theme="@android:style/Theme.Light.NoTitleBar" />

    <receiver android:name=".com.taxeeta.HomeScreen.OnAlarmReceiver" />
</application>

Output of adb shell dumpsys alarm

 com.taxeeta
    51471ms running, 5248 wakeups