Captions with Shaka Player

using shaka player I couldn't add a track to player nor manually neither programmatically.

Manually

placing <track src="src" kind="subtitles" > inside <video>

Programatically

using shaka player addTextTrack(uri, language, kind, mime, opt_codecopt, opt_labelopt) method

In both ways, the vtt file is fetched from the server but it doesn't show anything. using first method cause a cc icon appear next to the volume control but it doesn't do anything.