That cold shock at the end of a shower line
If you have a house full of people and mornings feel like a relay race, you already know the pattern. First shower is fine. Second one is warm but fading fast. By the third, whoever drew the short straw is hopping around under lukewarm water wondering if the heater is broken. It usually isn't broken. It's undersized for what your household is actually asking it to do.
This shows up a lot in Nixa, especially in the newer subdivisions off Highway 14 and around Aurora Street where three and four bedroom houses went up over the last fifteen or twenty years. Builders often installed a standard 40 or 50 gallon tank sized for the house's square footage, not for the number of teenagers who'd eventually be showering out of it every morning before school.
What's actually happening inside the tank
A tank water heater has a recovery rate, the speed at which it reheats water after it's been drawn down. Every gallon you use gets replaced by cold water from your supply line, and the burner or elements have to catch back up. If your showers are spaced out, the tank usually wins that race. If two or three showers happen inside twenty minutes, it often doesn't.
Winter makes this worse here. Groundwater temperature in southwest Missouri drops noticeably from December through February, sometimes into the 40s, compared to the 60s in summer. That colder incoming water needs more energy to heat, which stretches recovery time exactly when your household is layering up and taking longer showers anyway.
More on this from Sizing A Water Heater For Back To Back Showers.
What you can check yourself
Look at the data plate on the side of your tank. It lists the tank size in gallons and, on gas units, the first hour rating, which tells you roughly how much hot water it can deliver in the first sixty minutes of use from a full tank. Compare that number to a rough estimate of your household's morning demand: about 10 to 15 gallons per average shower, more for a soaking shower head or a teenager who showers like it's a hobby.
Check the age while you're down there. Most tank units in this area last eight to twelve years, and older units lose efficiency and recovery speed as sediment builds up in the bottom of the tank. If your unit is pushing that age and your household has grown since it was installed, sizing problems and age problems are probably stacking on top of each other.
You can also try a simple test. Run the dishwasher or a load of hot laundry right before the morning shower rush and see if that makes things worse. If it does, you've confirmed the tank is being asked to do more than it can keep up with, not just failing on showers alone.
Where this stops being a DIY question
Deciding whether to size up a tank, switch to a higher recovery gas unit, or move to tankless is not a project to guess at. Tankless units solve the back to back shower problem by heating water on demand instead of storing it, but they need to be sized to your home's flow rate and, in this region, sized with an eye toward how cold that winter groundwater actually gets. Undersize a tankless unit for a cold Ozarks winter and you'll get the same lukewarm surprise you're trying to fix.
Gas line capacity, vent routing, and electrical requirements for a larger unit are also not something to eyeball. A house built in the late 1990s or early 2000s around Christian County often has gas and electrical service sized for the original smaller heater, and swapping in something bigger without checking that first can create problems that have nothing to do with hot water.
What's reasonable to expect from a fix
A properly sized system, tank or tankless, should get a household of four through a normal shower rush without anyone getting the cold shoulder. It shouldn't require staggering showers by twenty minutes or running the water heater at a higher, less efficient temperature just to stretch supply. If your current setup requires either of those workarounds, that's the system telling you it's undersized rather than failing.