Have you already maxed your Inscription skill? The Minor Inscription Research will only fail if you have already learned all available glyphs. However, some minor glyphs have a higher level (represented by the skill level at which you can gain skill by scribing them) which means you can't discover them until your inscription is at least as high as their skill level.
Currently there are 64 minor glyphs, so it might also help to count and verify if you have them all. Unfortunately, Wowhead's search function isn't properly differentiating between minor, major, and prime glpyhs, so I can't give you a better answer.