Low water pressure from tub spout- diverter works fine
Your description is too short of needed facts surrounding your problem. You need to ask yourself.....self, has there been any thing that could have effected plumbing? Did my water pressure suddenly drop? Did someone just do some work on my water piping? Is my water pipe under the house? Have I smelled any musty smells recently? If it is only the hot water....check the shut off valve on your water heater to make sure it is all the way open. Are any other faucets affected? or just the hot on your tub? If your Delta tub valve is the older type....usually installed in the 70's or 80's, the old type that has a stainless ball that operates by moving the handle up then twisting the handle right or left to get more mix of either hot or cold water? Because if it is that old version, they are notorious for symptoms just like yours. When the inner parts become old and loose, the ball inside the valve won't rotate all the way to allow hot water to flow freely through the port in the ball because it will no longer turn to the point where the ball port and the supply port match up. If you have this old valve, it is very rebuild-able, but if your a novice, do some research before tackling it yourself. It's rebuild-able.....but a little on the difficult side. If you have under floor water piping, and have access to your under floor, take a look and see if there might be a water leak. And last thing is to disassemble your valve and check it for obstruction. It's not uncommon for debris from your water heater to be picked up by the flow of water through the tank then being lodged in the smallest port on the most demanding fixture in your home. Which is the tub or shower valves. Again, not an easy peasy task to perform, so if your a newby, study real hard before you attempt to disassemble your tub or shower valves, that means get a positive ID on your valve and download a parts break down picture to study all about it. What parts are interchangeable and where you can get the part. It would help your cause and prevent us, the answer guru's, from pecking our finger tips into oblivion typing novel like answers to questions, if you would give a better description of your situation. Because the answer to your simple stated problem is impossible to pin point in it's current condition of lacking substance. I say this in all kindness, so don't think I'm being rude, I just trying to help you get your problem fixed. If your out of your league all together, my fellow Plumbers would like for me to endorse your hire of one of us to fix you up. We can always use the work. Good luck with solving your problem, remember that you should always know where and how to turn your water off to your home before attempting a plumbing repair, and always make sure to turn your house water off before proceeding to disassemble your tub or shower valves. Both of these valves are hard piped and do not have a way to be shut off ....like your faucets can be shut off under the counters. :-)