Central air not cooling (or maybe it is?)

I purchased this home in Nov. of 2014, its a old house built in the 50's but everything from the floor joists and up is brand new, including the HVAC. Entire house is 1600 sq. ft., estimating basement + first floor is 1100 sq. ft. and the 2nd floor is 500 sq. ft. My air units are good for up to 1200 sq ft each (I have 2). One unit does basement + first floor, other unit does 2nd floor. First floor + basement = 12 air vents 2nd floor = 5 air vents So my issue is, the 2nd floor heats and cools like a charm. Air comes out of the vents like a jet engine. However my first floor/basement is where my family spends 90% of the time, so less air gets pushed around to each of these 12 vents. These last 4 days I just started using the air conditioner in the evenings... my thermostat (with windows open) will show it being 70-75 degrees from morning until early afternoon, and then once the sun gets over the house is when it jumps up to 80. Around 78 degrees if when I shut the windows and cut the AC on. The AC cuts on around 2pm and is set to 77 degrees, and never shuts off. Keep in mind the high temperature from each of these days is no higher then 82.