The subtitle is probably corrupted or in a format that isn't recognized by Freemake Video Converter. I've checked the official website and it appears that you can add the following subtitle formats to the movies you already have: SRT, ASS, and SSA.
I suggest checking the file with a text editor because the SRT files are in plain text format. If you see weird characters in the file, then it means it was corrupted and you need another version of the subtitle.
