Are you referring to textbased .sub-file or graphics based .sub-file ?
What is size of the file and/or can you read it using e.g. Notepad?
(The size of text-base subtitle-files usually is about 20-50 kB. If the size of a .sub-file is substantially larger (in MegaBytes as opposed to KiloBytes), it concerns VobSub (DVD) subtitles in a graphic (picture) format).
(You specifically mention .sub, so I assume .srt-subtitles do show.)