Why Subtitles Don't Appear
Both PGS and and VOBSUB subtitles consist of a series of bitmapped images and aren't actually "text" at all. PGS subtitles on a blu-ray are 1920x1080px resolution images, for instance, which is a 16:9 aspect ratio.
In many cases, the video content will actually be a different aspect ratio, such as 1.85:1 or 2.35:1. When making the blu-ray, black letterboxing is added to the video so that it ends up as a 16:9 ratio 1920x1080px video when rendered. When content is ripped from a blu-ray, the letterboxing is typically removed so that only the video itself remains, but this can result in a video file with an actual resolution of 1920x800px, for example.
When the subtitle image (1920x1080) is overlayed on top of the video (1920x800), the subtitle image is larger and thus part of it extends beyond the actual video. The area that extends beyond the video is effectively lost and won't show up. Unfortunately, this part is typically at the bottom and usually contains the actual subtitle text, thus the subtitles might not appear at all because they're "off screen".
What Can I Do?
The easiest thing to do is simply get another subtitle for the item in question. There are numerous websites devoted to providing subtitles for movies and TV shows. These sites most commonly use SRT format subtitles, which are text files and don't suffer from the same sort of issue.