The hot water faucet in my tub has lost pressure. It barely lets water through. My sink is fine. My faucet is two knobs mounted to the wall, the tub spicket, and an external hand-held shower head. The name on the knobs is Rockwell. The spicket was recently replaced.

Chances are when the faucet was replaced, some crud was knocked loose and ended up blocking the hot water. Depending on what style it is, you may be able to pull the valve stem and get the blockage out. Removing the feed pipe may also give access to a plug. Just keep checking the hot water passages until you find the problem.

Labman analyzed your problem corectly.  Let me carry it a step further and tell you how to open up the valve and flush out your supply lines. Turn off the water. You will then unscrew the screw securing the hot water knob and pull it off. Next unscrew the knured nut securing the cover plate and pull it off. You will be left with a stem coming out of a brass nut. This nut is called the bonnet packing nut. Unscrew the nut but leave it on the stem. Now put the knob back on temporally and back the stem and washer assemble out counter clockwise.  Check the seat for trash and with the valve body still open turn the water back on for a moment . Strong stream?  You are flushing out the hot water supply line and clearing the valve.  Replace the washer if worn and reassemble.  Good luck,  Tom