How do I get full pressure to shower head?

When I pull up on the knob on the bathtub spout to send the water to the showerhead, there is still water coming through the tub spout as well as the showerhead. As a result, this is reducing the pressure out of the showerhead. We are selling our house and the prospective buyer would like it fixed. It's been this way since we had it and it never really concerned us. Could you shed some light on what the problem might be and how I could get it fixed? Thanks, Greg Paxton