What does the Nest's display show that it is trying to do? Does it show that it knows it's supposed to cool to 73, and that it knows it's hotter than that currently? Does it show that it's telling the AC to run? Is your compressor running?
Without more information it's impossible to tell whether the problem is in the Nest's programming, or the Nest's communication with your AC, or the AC system.
I wanted to check in and see if you managed to see Edward's post. Please let me know if you have any questions from here. I would be happy to assist, and make sure you are good to go.
I want to ensure you are good to go, and everything is working properly. Please let me know if you are still having trouble, as I will be locking the thread in 24 hours due to inactivity.