I recently purchased a house that was flipped (that was my first mistake) and the 2nd story bathroom was added altogether, and the shower has always leaked into my kitchen. Finally, 9 months, 4 people, and lots of money later, it is fixed. But now the problem is that I can't really get hot water to come through. My water heater is in the basement, so I expected it to take a little longer to get hot, but I just let it run and it was warm-ish for a minute but finally turned cold before ever getting to be as hot as I would like. The strange part is that both sinks in the same bathroom will get hot. My hot water heater is turned up as high as it will go. Any ideas? I have taken a shower up here a few times, just when my plumber would claim it was fixed, and I was able to get hot water then. However, that was a few months ago so it wasn't quite as cold outside then. Not sure if that's relevant. I'm pretty desperate to get a working shower on the same floor as my bedroom, so any advice is appreciated.
SInce you know hot water is making it to the 2nd floor, there are pretty much two possibilties. One, as TPE suggested, is that you have a thermostatic shower valve that's not working -- or even a mixer valve that isn't opening on the hot-water side. The other is that the shower is on a separate hot-water feed line from the sinks, and that this line is kinked, or has a cut-off valve somewhere that is shut or jammed (or is just letting a trickle of hot water through.
I fear that without being on-site none of us can tell what your plumbing "map" is. My usual advice is to ask your friends who they have used and recommend for plumbing work, and explain in the same detail as you did here what's wrong.
If you are sure you are waiting long enough for the hot water to get there, it may be an issue with the mixing valve for the shower. With water it either flows where you ask for or it does not, so if you are getting some hot water, then the piping should not be the issue but likely the valve itself.
Another potential problem besides a theromostatic valve is an anti scalding valve. This would be attached to the pipe behind the shower, and is designed to prevent the water from getting over a certain temperature. I've had them fail from mineral buildup where they would only allow cold water to pass through.
This is something your plumber should catch. If you paid for his service and he said it was fixd call him back to have him fix it right. If it's the shower valve he should be able to identify that problem very easily. You tell by takeing trim ring off. Run the fixture. Feel hot inlet with finger after running for minutes. If hot problem with the shower valve. If only warm. Other problem.