For what its worth, a 20° temp drop for the air over the evaporator is not, in and of itself, proof of a properly functioning system. The temp drop is a variable commodity based on a few factors, primarily:
1- The latent heat load of the air across the evaporator.
The greater the relative humidity of that air, the less the temp
drop. As the AC removes the latent load, the temp drop will
increase. As MEX posted, latent load takes priority.
2- The volume of air across the evaporator.
The greater the CFM of air, the less the temp drop. A dirty coil
or filter would increase the temp drop.
If you have not done so lately, now would be a good time to clean the condenser as well.
