+1 this. It may simply need a spacer under the locknut to prevent the shoulder on the locknut from bottoming out on the end of the steer tube. Failing that, check the threads on the aluminum locknut and threaded cup. They can get damaged easiy by the harder steel steer tube if they are ridden while loose. Eroded threads on the steer tube itself are another possibility, but this damage would have happened earlier, if a steel headset was used before the current one.