Air and moisture inside A/C system.
Moisture that enters the A/C system when is serviced or any component has been replaced will be harmful to the system and cause poor auto air conditioning performance.
Moisture cannot be removed from the A/C system after charging the system with new refrigerant. Moisture that enters the A/C system will be harmful to the system and cause poor performance because of internal erosion and the possibility of the moisture turning to ice near the expansion valve.
Fix the air and moisture inside the auto air conditioning system.