Are we talking about SRT subtitles?
If so, make sure that these subtitles don’t contain “overlapping” subtitles. Meaning the time stamps of two lines would require them to be shown at the same time. This doesn’t work in many devices due to limitations of their SRT renderer.
You can check for and correct issues like these with Subtitle Edit. See Introduction: convert image-based subtitles to SRT with Subtitle Edit
