Low Pressure

Since the problem is only on the hot side, then the obstruction must be in the hot alone (rust, dirt, whatever it is. It's either in the stop valve, the supply tube, or in the faucet itself, before it joins with the cold. Here's how to proceed. If its a two handle faucet, shut off the water and take the hot side apart. The problem might be obviously right there. If that doesn't work, put a rag over the exposed hot side, and put your hand over the spout, and slowly turn on the cold water. This will prove whether the path is clear from the spout to the hot stem. If it's a single handle faucet, still take it apart. You may find the problem within reach. If this doesn't work, then the problem may be in the supply tube itself, or where it attached to the faucet. Take off the hot supply line off and blow through it. If it's OK, there's a chance the obstruction is somewhere in the faucet. Try backflushing it by reattaching the supply at the faucet, putting your hand over the spout, and forcing water through the faucet by slowly turning on the cold water. If that doesn't work, the problem is very likely in the stop valve itself. This will mean you'll have to shut off the water heater inlet and take apart the stopvalve itself, and clean it. These things are common, and it'll be obvious once you find the trouble. If you fail miserably, write back with the name and type of faucet.