A/C Runs non-stop and shuts down without reaching desired temp
Here are some things that you can do:


check the air filter, could be frozen or dirty enough to restrict airflow.
check for any possible ice build-up
shut down the system from the breaker for 30 minutes: this will reset the system and allow time to melt any ice build-up. Power back on and see if the temperature holds.


Here is what I would recommend to have a professional do


Verify that the thermostat is working correctly. You didn't seem 100% on this point and a bad thermostat would give the results you are getting.
Have the pressure re-checked. Too much or too little coolant can affect the efficiency of the unit.


Get a second opinion

The issue should have been resolved by a professional within 2 visits. If it takes more than that, you need a fresh pair of eyes to look and re-assess the problem(s) with the unit. Even if you need to pay an additional fee with someone else, it might be worth it to get the correct resolution.

If the unit is under warranty, you can request from the company responsible to send a different technician for a second opinion at no charge.