The unit does not reach set temperature. What should I check?

With 12v portable fridge freezers, make sure that the voltage going to the unit is constant and above the start-up voltage depending on the voltage cut out selected.
Make sure that the condenser is clean and dust-free.
Make sure that there is sufficient ventilation around the unit. The temperature may vary by a few degrees depending on ambient temperatures.
Check the condenser fan to make sure the compressor is not overheating.