If you live in a household with multiple people, you're going to have differences, and the preferred hot water temperature is one of the most likely issues to come up. Sometimes people just don't think it's important to have really hot water, and they'll set the water heater to a lower setting. But others want that hot water because to them, a hot shower is also stress relief, or maybe they have a lot of laundry that needs to be washed in very hot water. Or maybe there's a small child in the house, and scalding is a risk. There are solutions that let people have the hot water they need.
Evaluate the Plumbing and Current Heater
First ensure that everything is working properly. Someone complaining about showers that won't get hot might be trying to shower at the same time that someone else is doing a load of laundry on the hot cycle as well. Or there could be a problem with one of the pipes leading to a fixture.
The water heater should also get a checkup, especially the thermostat, to ensure there are no problems. It could be that the heater is old and is shutting down intermittently, creating a problem for anyone who regularly tries to use hot water.
Also note where the complaints are happening. If it's people on the top floor of a house with the heater in the basement, chances are that hot water just isn't making its way to that top floor before cooling off. In that case, installing a second water heater on the top floor might be helpful.
If all seems to be well on the mechanical side, try making temporary adjustments. For example, if most of the household wants water at a certain temperature, but you want hotter water for showers, see if you can raise the temperature for just the time when you're taking the shower. Raise it a few minutes before you shower, though, so water in the lines and tank (if you have a tank-style heater) has time to warm up more. Otherwise, you're going to have to work your way through the cooler water first.
Dedicated Tankless Heaters
Another option is to install a tankless heater that serves just the fixtures that are used by the people who want hotter water. For example, you could have a tank that serves most of the house, but one tankless heater dedicated to the washing machine. Or, in the previous example where people weren't getting hot water on the top floor of the house, one tankless heater upstairs can be enough to solve the problem.
Start by calling a water heater repair company to check out the appliance and pipes. The employees will let you know if there's a problem that needs attention.