One problem here, the ducts accessible from the basement are all Heat ducts. The AC ducts run through the rafters in the ceiling.
Pull one of those vents and using a flashlight and mirror check that the AC duct isn't blocked. Some have had problems with these cheap cardboard ducts not keeping their shape and restricting air flow.
Michelle & Ann
2018 Chevy 3500HD Crew Cab High Country DRW, D/A, 2016 RW39MB, Dual ACs, Auto Level, Auto Sat Dish, Stack W/D, King Sleep #, 17.5" Sailuns w/Disc, MORryde IS & Pin, Comfort Ride Hitch, 5.5 Onan, Res Fridge & Induction Cook Top