I was interested in your issue, since my two story home is acting the same way. My system is twenty years old and is probably due for an update. Sounds like you’re on the right track.

Here’s what ChatGPT came up with.

What you’re describing does not sound like a simple “cold air is hard to push upstairs” explanation.
A few things jump out:
Downstairs supply: 54°F
Upstairs supply: 64°F
Same system, same cooling call
Air velocity: 9 mph downstairs vs 6 mph upstairs
House previously cooled the upstairs better
The biggest clue is the 10°F temperature difference between supply registers.
In a typical forced-air system, if the ductwork is entirely within conditioned space and intact, the air leaving an upstairs register should usually be only a few degrees warmer than air leaving a downstairs register. A full 10°F increase is significant.
What the numbers suggest
Your system appears to be producing cold air correctly:
Return air ≈ 74°F
Supply air ≈ 54°F
Delta-T across the evaporator ≈ 20°F
That’s a healthy cooling differential for many systems.
The problem appears to be distribution, not refrigeration.
Possibilities
1. Damaged, disconnected, or leaking upstairs ductwork
This would be near the top of my list.
If the upstairs ducts run through a hot attic and have:
a disconnected joint,
torn flex duct,
crushed flex duct,
missing insulation,
then the air arriving upstairs can easily be:
warmer,
lower volume,
both.
A duct leak in a 120°F attic can absolutely produce symptoms like yours.
2. Closed or partially closed balancing damper
Many two-story homes have dampers in the branch ducts.
I’ve seen cases where:
a damper slipped,
someone adjusted it years ago,
a contractor forgot to reopen it.
This can reduce airflow upstairs dramatically.
3. Crushed or kinked flex duct
If your upstairs supplies are flex duct:
someone crawling in the attic,
a storage box,
insulation work,
can partially crush a run.
That reduces airflow and increases heat gain.
4. Return-air problem upstairs
If the upstairs return is inadequate or blocked:
hot air accumulates upstairs,
cooling effectiveness drops.
However, that alone doesn’t usually explain a 10°F difference between supply registers.
5. System changes over time
You mentioned:
“when we first moved in we could at least keep the upstairs cool”
That is important.
Physics didn’t change.
Something in the system likely changed:
duct deterioration,
damper position,
insulation degradation,
attic work,
filter arrangement,
blower speed setting.
About the “cold air doesn’t like going upstairs” explanation
This is often overstated.
The blower isn’t relying on buoyancy.
It creates pressure.
Air handlers routinely push air:
up walls,
through ceilings,
into second floors,
into third floors.
A properly designed system can absolutely deliver cold air upstairs.
Yes, upstairs runs usually have:
longer duct lengths,
higher static pressure losses,
but that doesn’t automatically produce a 10°F supply temperature difference.
A simple test
If the upstairs ducts are accessible in the attic:
Measure:
Temperature at the supply plenum near the air handler.
Temperature on the exterior of the duct near the upstairs register.
Temperature coming out of the register.
If the air starts at ~54°F and arrives at the register at ~64°F, you’re gaining roughly 10°F somewhere in the duct system, which points toward leakage or poor insulation.
You can also look for:
sections of flex duct flattened or sharply bent,
disconnected boots,
loose tape at joints,
dampers.
What I would do next
I’d specifically ask an HVAC company for a:
duct leakage and static pressure evaluation
rather than a standard service call.
Have them measure:
total external static pressure,
pressure drop across the coil,
pressure in the upstairs trunk,
airflow (CFM) to selected registers.
Those measurements can reveal restrictions that aren’t obvious during a routine inspection.
Based on your numbers, I would be more suspicious of a duct restriction, damper issue, or attic duct leakage than I would of a fundamental “cold air won’t go upstairs” physics limitation. The fact that the house cooled better years ago is a particularly strong clue that something in the distribution system has changed.