There are a few reasons why your AC may not be working on one side. The most common reason is that the unit is not properly balanced. This means that the refrigerant level in the unit is too low or too high, which prevents proper cooling. If this is the case, you'll need to have a professional come out and service your AC.
Another possibility is that there's an issue with the condenser coils. These coils help to transfer heat from inside your home to the outside, so if they're dirty or damaged, it can prevent your AC from working properly. Again, you'll need to have a professional come out and take a look at your unit to diagnose and fix the problem.
Finally, it's possible that there's simply an airflow issue. This could be due to a blockage in the ductwork or vents, or something as simple as a closed register. You can check for yourself whether this is the case by feeling around each vent to see if air is coming out evenly on both sides. If not, then you'll need to investigate further to find the source of the blockage and get it cleared so that cool air can flow freely again.