When the stopper is pulled up, the shower works, but there is still water coming through the spout in the tub. How can I stop this water from going into the tub and redirect it to the showerhead?

Q: I have a bathtub/shower combination, and when I pull the knob on top of the spout to start the shower, only a little water comes out of the shower head while most of it comes out of the tub spout.