Make sure to check if the game that you are playing has the feature where it only shows mods that are compatible with the game's current version.
In Sins of a Solar Empire, for example, they implimented a program that makes all out-of-date mods invisible on the in-game mod screen, thus helping prevent game crashing problems due to trying to load incompatible mods.
Also make sure to check that the mod you want to use is compatible with the version of the game that you have.
-Dakroth