I had this same problem with mine. I kept thinking it was hair and gunk stuck to the linkage and weighing it down etc. I would take it apart, clean it, but it really didn't do the trick. I finally wised up and realized that it was the drain flipper that needed replacing. Go to your hardware store/home center and purchase a replacement. These are usually generic parts that you can get almost anywhere. You want the whole assembly that has the two screws on it, and the flipper arm. You will notice that the spring in the back is firm and it stays positively in the upper or lower position and that your old one just flops around.