Does it have a return cable or just a single one? If two cables, ensure that the return cable is loose, it doesn't need to be anything like as tight as the pull cable and if it's too tight it'll cause a lot of drag. Otherwise, as Thunderbird said, check all the routing carefully and ensure there are no tight bends or pinch spots - with hindsight, tying a piece of string to one end of the old one and pulling the new one through with it might have helped :