I’m surprised no one mentioned this so far: check to make sure the fins on your outside unit are clean.
My parents had this issue when I was visiting. I went outside and looked at their unit and it looked like it was covered in carpet. I took a hose and gently rinsed off all of the dirt and pollen and they were back in action.
Others already covered a lot of good points: make sure it’s not froze (it likely is if you’ve been running for a while and it’s not keeping up) and change the filter.
If that doesn’t work you likely need refrigerant.
As others have stated, replace your air filter if you haven’t recently. Make sure it’s the appropriate size and rating. It should probably be pleated, and the MERV rating shouldn’t be too high unless you know the fan supports it.
Use a temp gun or a kitchen instant read thermometer and compare the exhaust temp vs the return temp. It should be around 15 degrees difference or more. If it’s not then an installer should adjust the pressure on the system.
If you have multiple floors you may have a vent damper for summer vs winter. In the winter the dampers may be set to help force air through the lower floor vents and prevent the hot air from rising too fast to the upper floors. If the installer or previous owner was nice the damper will be labeled for summer vs winter.
If it’s not blowing air at all, then you may have a leak or a blockage.
If theres ice on the indoor coil or on the lines outside then either the filter is clogged or your pressure is off.
If your hvac person is telling you it’s working fine and it obviously is not, call a different installer.
when this happened to me the heat exchanger inside was all iced up, turned out it was a valve in the circulation. Couldn’t fix it myself due to compressed coolant.
The other posters have given good info. I just wanted to add to try setting the fan to on instead of auto so it runs all of the time. This will help keep the air moving in the house and might help keep it cooler. Also make sure you check the filter and change it if you haven’t yet. See if your energy company offers an energy audit. that will tell you if you have leaky windows, doors, or insulation problems. I have shitty insulation but with the ac fan running all of the time I can keep the house @ 70 - 72 on a 95 degree day.
I’d also consider getting a second opinion from another HVAC company, you could have undersized ductwork or another problem that the first company isn’t acknowledging.
Two issues with running the blower all the time. One, wear and tear on the blower over time. More importantly though, the coils in the unit need occasional downtime to let moisture drain for best efficiency. Your points on moving air while not cooling are valid though, and newer thermostats do have a circulation mode that will cut on the blower for a percentage of the time the AC itself isn’t on to do move air around. Good for second story homes with one AC, as the thermostat is usually on the first floor while the heat rises to the top floor.
I’m a layman but from what I’ve read recently, 92 to 78 is 14 degrees; the typical temperature differential is a maximum of about 20. There may be a few more degrees you can eke out, but you are getting a majority of the potential benefits