It's 12K BTU/hr, and you've given it an uninsulated wall in the sun (R1, MAYBE) and a 20 degree differential, perhaps more due to sun striking the wall/roof. 600 square feet of uninsulated area would use all that up at 20 degrees differential. Given an uninsulated ceiling that's probably 400 square feet (average two car size, since you haven't clarified) and an uninsulated wall, hardly a wonder that you're maxing it out. The "insulated" walls are not perfectly insulated, so there's more load, but you've not given anywhere near enough information to detail how much load they add.
I use a pair of 12K units in a 2000 square foot building and they (or even just the upper one) cool it trivially to 68F at 90+ outside - because it's well insulated - R35 or so for walls and roof, R17 for the garage door. I only have two because they also heat it, and that's a bit more work (-15F out and 65F in has been seen.)
You need at least SOME insulation everywhere (because where you have none, even a little bit will cut your losses by a huge factor - going from R1 to R5 is 20% the load.) That's a very large return on not much insulation. Getting it up to something halfway respectable like R19 will let the unit cool the place effectively AND save you a good deal on your electric bill.