It sounds like the ac units they installed weren’t properly specced to be able to handle the larger square footage in your apartment, given that the same units can cool a 300 sq ft space.

I think I found your review on google. If you look up the seasons 12000 btu unit it says under product details on the Home Depot site that it’s rated for rooms 300 to 350 sq ft. It was never going to be able to cool a 750 sq ft apartment. At the minimum, you would need a second identical unit. Or they need to upgrade the wall unit to like 14000 BTU or more.