Air Conditioning - CAC all charged and inspected, hard time cooling

I'm in NJ and I have a 16 year old CAC that last August ran out of cooling capacity. One day it couldn't even get an 80 degree day to 75 even if it ran for hours. I had a CAC technician come in to inspect the unit and the only thing he found was it needed 2 pounds of freon. So I assumed fill it up and I should get through most of the summer again. Well that doesn't appear to be the case. It seems once again to be having problems. We had a very hot day today high 80 and it struggles to get the temperature in the house from 80 to 76. Basically ran for 4 hours and made 76 when the outside temp was 75.

I put an indoor thermometer in front of one of the supply ducts and the temperature registered 65, about 12 degrees cooler than the house. What is the temperature supposed to be? I thought I read 20 degrees differential somewhere.

Second question is, what else could be wrong with a CAC where it functions and has all its freon but can't cool the house down like it used to? Thanks.

The leak should have been found and repaired before refrigerant was added.
To loose two pounds since last August should make for a fairly easy find.