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.