The last time I had an A/C system tested for leaks, a nationwide mobile service filled the system with nitrogen and looked for leaks using an instrument that was sensitive to refrigerant! Of course, he then claimed there weren't any leaks, filled up the system with refrigerant and departed. A few days later I was back to square one! Check for leaks by looking for oil deposits. The likely source is the condenser because of its' light construction and large surface area. Change the dryer at the same time.
Another possible cause is that the pressure sensor has failed, such that refrigerant is present but the sensor reads zero and therefore prevents the A/C from working. You should be able to change the sensor without emptying/refilling the system.
RAB