That filter you upgraded is working against you
If you swapped your old filter for one of those thick pleated ones with a high MERV rating sometime in the last year or two, and your system seems to be running longer, your upstairs is staying warmer than it should in July, or you hear more air noise at the vents, that filter is probably the reason. It is not a coincidence and it is not your imagination.
A lot of homes around Nixa, Ozark, and out toward Highlandville are older ranch and split-level houses built in the 70s, 80s, and 90s, with ductwork and return sizes designed for a standard one-inch fiberglass filter. When someone decides a higher MERV number must mean cleaner air, they drop in a filter that's rated for hospital-grade filtration without changing anything else about the system. The equipment wasn't built to pull air through that much resistance.
Why this matters more here than in a lot of places
Southwest Missouri puts HVAC systems through real extremes. Summers are hot and sticky, with enough humidity off the Ozarks that your AC is doing double duty, pulling moisture out of the air as much as cooling it. Winters swing into real cold snaps, sometimes with ice, which means the furnace side works hard too. A system fighting a restrictive filter in both seasons gets no break.
When airflow is choked down, the air conditioner's evaporator coil can get too cold and start icing up, even in August. The furnace side can overheat its heat exchanger and shut itself off on limit switches. Neither of these is a minor inconvenience. Both wear out expensive parts faster than they should, and both tend to show up as a breakdown on the first truly brutal day of the year, which in Nixa is either a 100-degree afternoon or a single-digit night.
More on this from The High-Merv Filter That Costs You More Than It Filters.
What you can check yourself
Pull the filter and look at the frame for a MERV number. Anything at MERV 13 or higher in a system that was never upgraded for it is worth a second look. Check if your return air grille is a single small cut-in, common in older Nixa homes with one central return, or if you've got multiple returns. A single, small return pushing air through a high-MERV filter is the classic setup that causes trouble.
Feel the airflow at your supply vents on a day when the system has been running a while. Weak, lukewarm air blowing out, especially in rooms farthest from the unit, is a sign the system is starving for air. Look at the filter itself after a month. If it's still bright white and barely dirty, that can mean air is finding the path of least resistance around the filter edges rather than through it, which defeats the purpose anyway.
Check your thermostat's run times if it tracks them, or just notice if the system seems to run in long stretches without really catching up to the set temperature. That's a classic symptom of restricted airflow, not just summer heat.
Where this stops being a DIY fix
Switching back to a lower MERV filter, something in the 8 to 11 range, solves the immediate airflow problem for most standard residential systems and costs nothing but the price of the filter. That part you can absolutely do yourself.
What you can't eyeball is whether your system's duct sizing, blower motor, and return setup could actually handle a high-MERV filter if the ductwork were modified for it. That takes a technician measuring static pressure with a manometer, not guesswork. If you've had repeated frozen coils, a furnace that trips on high limit, or a blower motor that's already been replaced once, those are signs the system has been struggling with airflow for a while, and it's worth having someone check the whole setup rather than just swapping filters and hoping.
What to ask for during a regular tune-up
If you're already having your system serviced twice a year, which is a reasonable habit in this climate given how hard both seasons push the equipment, ask the technician to check static pressure while they're there. It's a quick test and it tells you definitively whether your current filter, whatever its rating, is within a range your ductwork and blower can handle.
This matters more in Nixa than in a mild climate because the margin for error is smaller. A system that's slightly restricted might limp along fine in a place with short, mild summers. Here, where the AC runs hard for months and the furnace gets tested by real cold snaps, a restricted system doesn't just underperform, it tends to fail at the worst possible time.