Embedded Subtitles not working on some movies

Hi All,
I'm trying to get embedded subtitles from movies i've removed the DRM from to work on Plex, I've made sure the subtitles work after removing the DRM (software i use keeps the subtitles) and whenever i upload movies to plex with embedded subtitles, the subtitles don't work. Does anyone have any ideas on why this is the case and how i could possibly fix it?
I know the best and easiest option is to remove the subtitles completely and then put my own .srt files in it but the subtitles are usually wrong when i find them online and they are generally out of sync (i know i can resync them with other tools out on internet).
Thanks!