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.

Replacing a shower diverter problem is easy and inexpensive. If all goes well, there will be plenty of water coming out of the showerhead in about 30 minutes.

His next guess was the shower head—another place where water flow is suddenly constricted. Nope. Turns out the culprit was the diverter tub spout—that little knob you lift to direct the flow ...