Check that there isn’t a conflict between several mods. Usually, if it’s the case the MI will box the mod(s) in red. 
When your cursor is on the concerned mod, hit the “T” button (or the button below) to show conflicts (or use the button below).
Some mods require other mod(s) to be selected to work (dependencies). Make sure the needed dependencies are selected too. You can find them in the right part of the MI interface.
