When our tub spout diverter clogs, I use WD-40 with its spray nozzle curved up the spout to the diverter valve. A couple of sprays gets things loosened up and working again. Jackson Tutt ...

The tub spout you have with the knob on the end of it is a shower diverter. The knob diverts water away from the tub spout and up to the shower head. In your case, the diverter needs to be replaced.