I live in a townhome built in 1989, with the water heater on the third floor and the two bathrooms on the second floor.

In the master bath is a tub/shower, where the hot and cold water are controlled by a single knob that you turn all the way to the right to turn off, and the further you turn to the left, the hotter the water is supposed to be. The shower head is controlled by a valve stopper on the faucet that is supposed to switch the flow of water from the faucet to the shower head when you pull up on the stopper. Fairly standard.

The problem I have is that the further I turn the knob to the left, the less water pressure I will get out of the shower head. Water pressure is fine coming out of the faucet, like if you were going to draw a bath, but when I pull up on the valve stopper, the water continues to flow out of the faucet, and just really dribbles down from the shower head. When the knob is between the “H” and the “C,” water still comes out of the faucet, but enough water comes out of the shower for a decent shower, but frankly that is too cold. Turning the knob back to the “H,” and water pressure drops again from the shower head.

Any suggestions on how to fix this?

The other bathroom on the same floor has the same setup, but there are no issues with shower water pressure.

Thanks so much.

You have two problems. The first is that the diverter (what you are calling the valve stopper) is shot. These are not repairable but easily replaceable. To replace you simply replace the spout. Replacing the spout is fairly easy and not expensive.
There two types of spouts. There is the slip on type and the screw on type. Feel under the spout close to the wall. If there is an opening in the bottom of the spout you have the slip on type. To remove you have to loosen the set screw in that opening, that holds spout in place and then pull the spout off. The set screw is usually loosened with a screwdriver or a n Allen wrench.
If you don't have an opening under the spout you have the screw on type. It is removed by turning the entire spout, usually with a pipe wrench or a large pair of pliers.

The second problem you have is that it sounds like you have a restriction in the hot water side of the valve. The water coming out of the spout and the water coming out the shower head should be the same temperature.
There is only one pipe going up to the shower head, so the restriction must be in the valve itself. The cartridge in the valve probably need to be replaced.

Tell us what brand of valve you have. If you don't know or can't find the brand name, post a picture of the faucet. If we know the brand we can probably guide you through replacing the cartridge.

Replace the spout. If you don't have hot water out the shower head after replacing the spout, post back with the brand of the faucet or a picture.

Thanks. I appreciate the detailed answer. I will replace the spout, and follow up.