Subtitles on ht-f4500

I've done a search with no luck...

I'm trying to watch Dawn of the Planet of the Apes on my Samsung BD player ht-f4500 using a .srt subtitle file for the apes. It works for 5 minutes but then all subtitles flash up only for a split second so are impossible to read?

Has anyone else had this problem or know how to fix?

Thanks

An update. I played an mkv last night with embedded subs and they worked perfectly