Air Conditioning: Help! New A/C is not keeping up

I know this is long winded but I'm getting more than a little frustrated and I'm going to try and include as much info as possible. About 6 weeks ago I bought a new Trane AC and furnace to replace the 20+ year old unit for the upstairs of my house. The old unit still worked but had a slow freon leak and the furnace motor sounded like it was going to die anytime so I figured I would replace the whole thing on my terms and not wait for it to be an emergency. But this new unit is not keeping the house cool like the old one did. I've set the thermostat to hold at 74 degrees but by mid afternoon when the temperature reaches 90 or higher outside the inside temp will begin climbing until the sun goes down. The company that installed it has been out several times and found several issues, a boot they broke during the install, a loose duct, air leaks; and these have all been repaired but the problem is still present. They've even replaced the brand new condenser with another new one.

About my house, it was built in 1900 and there is no insulation. I live in NE Oklahoma.

All the windows though were replaced by the previous owner about 10 years ago with double pane windows.

It has a hip roof, the attic is a walk up attic and is all open with a floor. It has a ridge vent installed by the previous owner - he did not install soffit vents - in the summer I open the attic windows to ventilate.

The upstairs is about 1200 sqft.

The old unit was a 2.5 ton that used R12, the new unit is a Trane 3 ton 14 SEER (XB14 I think?).

The attic has a wood floor and I have no desire to tear out the floor to install insulation above the ceiling, especially on top of the old wiring. I am for insulating it to the roof though, its all pretty open and easy to get at.

Today at 4:00 PM it was 93 degrees outside, 114 degrees in my attic and 77 degrees and climbing inside. Thermostat was on 74 and the unit had been running nonstop since at least 2:00 PM.

The installers are starting to strongly suggest that the problem is the lack of insulation, and/or because they used my old ducts (which are insulated, R8 I believe) the service manager is going to have an insulation company come and inspect the house to see if they can figure out where all the cold is going.

I fully understand that having insulation will help with the overall efficiency but I'm lost as to why the old unit would keep the upstairs temp at 72 degrees even when it was 100 outside and the new unit can't.

Am I expecting too much thinking the new A/C should perform as well as the 20 yr old, smaller unit?

Would it help if I installed a fan to increase air flow through the attic?

What questions do I ask and what else do I look for?

Any advice would be appreciated... I'm working nights this week, if pics would help I can take some this afternoon.