With the information you gave it is hard to give you a proper diagnosis. This could be that you just missed your eye on the one or didn't hit it with enough of the drop to dilate. Or this could be a very serious matter that needs to be addressed immediately by a doctor. Has this just happened, is it a chronic problem? If you did get both of your eyes with the drops and only one dilated I would be getting in the car right now to the nearest ER. An eye that does not dilate properly shows that there is something seriously going wrong neurologically. Please go see a doctor asap!