Remove the stopper from the shower and examine it closely for cracks and tears. You can repair small cracks, holes and tears with a waterproof adhesive, but if the stopper is badly damaged, it is best to simply replace it with a new one.
Use a pair of needle nose pliers to gently lift up the strainer from the floor of the shower. Turn the strainer counterclockwise to remove it and then set it aside in a safe place where it will not be damaged.
Clean the hair, soap scum and other debris out of the pipe directly under the drain. Use a flat head screwdriver to remove the overflow plate and use a soft rag to clean inside the pipe.
Remove the plunger cup and apply petroleum jelly to the underside. Place it directly over the drain. Insert and remove it rapidly about 10 to 20 times. Let any standing water drain from the tub and then replace the overflow plate, the strainer and the drain stopper.