Your AC is running fine and the house still feels wrong
You've had the unit serviced. It's blowing cold at the vents. But by mid-afternoon in July, the upstairs bedrooms are noticeably warmer than the living room, and the thermostat is running almost constantly without quite catching up. People usually assume the equipment is undersized or getting old. In a lot of Springfield houses, especially anything built before the 1990s, the real problem is sitting right above the ceiling drywall and nobody's looked at it in years.
What Ozark summers and winters do to an attic
Southwest Missouri doesn't get the brutal, sustained heat of the Deep South, but it gets plenty of 90-plus days with high humidity, and attic temperatures here can climb past 140 degrees on a sunny afternoon. That heat sits right on top of your ductwork and your insulation. Then winter flips the problem: cold snaps and the occasional ice storm mean that same attic space is losing heat fast if the insulation is thin or compressed. The attic in this region works hard in both directions, and a lot of older homes were insulated to a standard that made sense decades ago, not to what the energy codes ask for now.
More on this from The Attic That Makes A Good Air Conditioner Look Bad.
What to check yourself
Pull the attic hatch or pull-down stairs and take a flashlight up there on a weekend. Look at the insulation depth between the joists. If you can see the tops of the ceiling joists clearly, you're under what most of this region's newer homes carry, and that's letting conditioned air bleed through the ceiling in both seasons.
Look at the ductwork if your system runs ducts through the attic, which is common in split-level and ranch homes built from the 1960s through the 1990s around Springfield. Check for duct insulation that's torn, sagging, or missing at the joints. Feel along the seams for cool air (in summer) escaping before it ever reaches a vent. Loose metal tape or crumbling foil-backed insulation are easy to spot and worth noting even if you don't fix them yourself.
Check for daylight. If you can see sunlight coming through at the eaves, soffits, or around any vent pipe, that's a path for unconditioned attic air to mix with your living space, and also a path for pests.
Look at whether the soffit vents are blocked by insulation that's been pushed or blown too far toward the eaves. Blocked soffit vents stop the attic from breathing properly, which makes the whole space hotter in summer and more prone to moisture problems in winter.
Where this turns into more than a weekend project
Spotting a problem in the attic is different from fixing it safely. Duct sealing and insulation work done wrong can trap moisture against roof sheathing, which in this climate, with our humid summers and freeze-thaw winters, leads to mold and rot you won't see until it's expensive. If you find disconnected duct runs, crushed flex duct, or insulation that's wet or matted down (a sign of a roof leak, not just age), that's a point to call someone rather than keep poking around.
Also stop and call for help if you notice your attic has almost no ventilation at all, no ridge vent, no soffit vents, nothing. Adding insulation to an attic that can't breathe just traps heat and moisture, and that's a setup that causes more problems than it solves.
Why this matters more than it seems to
An air conditioner working against a hot, leaky attic is doing two jobs: cooling the house and fighting a losing battle against heat pouring in through the ceiling. That shows up as higher bills, a system that short-cycles or runs long, and rooms that never quite feel right no matter how new the unit is. If you've already had an HVAC technician out and they mentioned duct leakage or suggested checking attic insulation, that's not a sales pitch, that's usually the actual cause. In Springfield's climate, with hot humid summers layered on top of cold winters, the attic is doing as much work as the equipment itself. It just doesn't get looked at nearly as often.