Measure the temperature of the return air at the A/C unit and the temperature at the discharge of one of the air registers. There should be about a 15 degree temperature difference if the A/C is working correctly.
Not maintaining interior temps can be due to an A/C not working correctly, but it can also be caused an A/C not big enough to handle the hot outside air temps. The above test can determine if the A/C is doing all it can do and is working properly.
Adding reflective films to windows can help a lot.