The system uses pressure switches that work to tell the system, "hey, good refrigerant pressure, we're good to work!" and "Hey, not enough pressure, don't engage the clutch so you won't do damage to the compressor." Say, you've got a system that will operate at 1lb. or more and our cars hold ~1.27lb. You could have a very small leak that has taken almost 10 years to leak out .3lb. of refrigerant so that the pressure switch sees that the pressure is just below it's operating pressure, so it won't allow the clutch to engage. You could add that .3lb and be okay for another ten years. OR! You could have a large leak that dumped every single bit of refrigerant out. Large leaks are easy to find with a leak detector or leak detector dye.