How is it possible to cool the air if it never passes through the evaporator? I think we both know what the answer is, it cannot happen. This tends to take place with improperly installed coils for A/C add-ons. But, cutting into the plenum to take a look is time consuming and for any mechanical company "time is money". Here is a way to be sure you should spend the time, just be prepared for what you will find!
The simplest way to know you have too much bypass air, air not being cooled by the evaporator through bypassing, is to measure the temperature drop (delta T) and superheat of the system during operation. If you have measured low superheat and low temperature split, then you have too much bypass air.
When there is a large amount of bypass air, Technicians tend to see low suction pressures and start to add refrigerant. This will not fix the system! In fact, you will likely flood the evaporator and run the chance of slugging the compressor with liquid. You must measure the superheat and subcooling to verify proper refrigerant charge. Do not fall into the pressure trap! Since the air is not traveling through the evaporator, the heat is not being delivered to the refrigerant and therefore cannot absorb that heat. This tends to drive the evaporator temperature lower, equaling lower suction pressure. If you try to add refrigerant, you could go through an entire 30# cylinder and not change your suction pressure!
Since the air is not traveling through the coil, the air is not being cooled, and you will measure a low temperature split (Return Air Dry Bulb - Supply Air Dry Bulb). Typically, temperature split on a properly commissioned system can be anywhere from 10⁰F to 30⁰F. The actual target temperature split depends on the operating conditions and proper refrigerant charge. When I say you will see low temperature split, you will most likely be under 10⁰F. This is not going to cool the building, under almost any load.
Add-on Evaporator with Bypass Air
Before spending the time cutting into plenums, redesigning duct systems, and cursing the installation crew, be sure to set the system up correctly. This means 350-450 CFM per ton of airflow (matched to the condensing unit), and an attempt to adjust the refrigerant charge. Then, if you find you still have low superheat and low temperature split, start cutting - cursing is really not needed and will not fix a thing!