Sometimes, mplayer won't recognize the format of a subtitle file.
In the console, it may be displayed:
SUB: Could not determine file format
Cannot load subtitles subtitle_file.srt.
Causes and solutions
For example the .srt file is encoded in UTF-16 when the system expects UTF-8.
A simple way to test the validity of the file encoding, is to do:
$ cat -t subtitle_file.srt
If you get some garbage output like: