if you're getting good pressure on both h&c water lines, then I have to assume that there is no restriction in your piping. Now we have to go thru some troubleshooting steps to attempt to isolate the problem; maybe one fixture at a time and the heater itself, since that's the common element; I realize you may have done this before, but it's important.
It sounds like you have a split level or upstairs home, is that correct and you are getting good pressure but no hot water upstairs, correct me if i'm wrong please, so that would make your bathroom on the same level as your kitchen, even if not, perform the following. If we can get this thing figured out, you can save a lot of money, even if we don't you will have saved having to pay a plumber to do the same thing. That would at least put in in the ballpark from the beginning & save him/her some time.
Ok here we go;
allow the water to run a good minute or so, unless you get hot water when doing the following;
I would like you to open the hot water valve only on the bathtub or shower and let me know if there is hot water there?
then the bathroom sink, but remove the screen first.
then the kitchen, but first remove the screen from the spout and run the hot water side only.
Let me know how you make out.