Exoplayer HLS doesn't show subtitle

I am developing a streaming app. I used for this purpose exoplayer2. I am using HLS streaming protocol. I need to show subtitle when streaming my videos. Everything goes well except subtitle. My player doesn't show the sample subtitle.
My code is as following:

uri = Uri.parse(url);
videoSource = new HlsMediaSource.Factory(DATA_SOURCE_FACTORY).createMediaSource(uri);
Uri uri1 = Uri.parse("https://bitdash-a.akamaihd.net/content/sintel/hls/subtitles_en.vtt");

Format subtitleFormat = Format.createTextSampleFormat(
null, // An identifier for the track. May be null.
MimeTypes.APPLICATION_SUBRIP, // The mime type. Must be set correctly.
Format.NO_VALUE, // Selection flags for the track.
"en");

DefaultBandwidthMeter bandwidthMeter2 = new DefaultBandwidthMeter();
DefaultDataSourceFactory dataSourceFactory = new DefaultDataSourceFactory(this,
Util.getUserAgent(this, "ExoCastDemoPlayer"), bandwidthMeter2);
subtitleSource = new SingleSampleMediaSource.Factory(dataSourceFactory).createMediaSource(uri1,subtitleFormat, C.TIME_UNSET);
MergingMediaSource mergedSource = new MergingMediaSource(videoSource, subtitleSource);
player.prepare(mergedSource);
player.seekTo(currentPosition);
player.setPlayWhenReady(true);
player.addListener(new IPlayerEventListener(this));
dropdown = findViewById(R.id.exo_resolution);
adapter = new SpinnerAdapter(this, items);
dropdown.setAdapter(adapter);
dropdown.setOnItemSelectedListener(new SpinnerOnItemSelectedListener());


I followed the developer guide of exoplayer to show subtitle but for nothing.
Thanks in advance.

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