Why is it that i cant sleep some nights?

some nights i go to bed and i just CANT sleep. i seriously just close my eyes for hours and never doze off. this will happen every so often but not all the time. i hate it. and also drowsy medication does NOT make me tired. when i get car sick i take dramamine which is verrry drowsy but nothing. this happens with alllll drowsy medication. what is wrong with meeee?