Make sure the shut-off valve under the fixture is opened all the way.

Check the valve so that debris isn't partially clogging the valve or pipe.

Remove, clean and repair any washers or seats by removing the hot stem.

Sometimes, its the piping itself. Galvanized water piping attracts calcium and minerals on the inside of the piping from the water. Water always has some concentration of calcium, you can't completely remove it, unless you distill it. Most cities simply filter the water and treat it for contaminants. But since calcium is not harmfull (except to piping) they typically only lower it to acceptable levels. After many years, though, galvanized piping becomes so clogged very little water can pass through it. This is because it reduces the inside diameter of a 1/2 inch pipe to sometimes less than 1/16th inch. This is especially true for hot water, which accelerates the process of extraction. the more active the water (hot) the quicker the process. The electrons in iron are constantly attracting the calcium. This doesn't happen in copper or plastic piping. So if you have copper or plastic, then there is another cause. Typically a valve or crimp.

Good Luck

Raymond VinZant Plumbing Prof.