Does the version of the mod match your game version. If you are running diplomacy 1.00 and the 7DS is for 1.01 you need to update the game. If your game is up-to-date, make sure the mod is updated for that version. The mods also have different downloads for vanilla, entrenchment and diplomacy. Be sure not to put an entrenchment mod into the diplomacy folder for example.
While some mods are stackable, lots are not. Check the forum threads for the mods you want to stack.
Some people, while extracting create a folder in a folder. When looking in your mod directory, it should look something like
...\Mods-Diplomacy v1.011\Bailknight's Graphic Mod\AllGAmeFiles
NOT
...\Mods-Diplomacy v1.011\Bailknight's Graphic Mod\Bailknight's Graphic Mod\AllGAmeFiles
