Subtitles not showing on USB in Toshiba fire tv 5OUF3D53DB

[Heddy]

I have just purchased a Toshiba fire tv 5OUF3D53DB. When I insert a USB into this - the film will play but will not show the encoded subtitles. The options available when this plays do not give a choice of subtitles. The subtitle button on the remote only works with the TV channels not with the USB. Can anyone help? Thanks.