When one room never cools, the instinct is that the system is too small. Usually it isn’t. Duct leakage in a typical Lowcountry home runs somewhere between twenty and thirty percent, and all of that conditioned air is going into an attic that does not need it.
Measure before you buy
A duct leakage test takes about an hour. Replacing a system takes a day and several thousand dollars. Doing them in that order is not complicated advice, but it is uncommon advice, because the second one is what gets sold.
What oversizing actually does
An oversized system cools the air fast, satisfies the thermostat, and shuts off before it has pulled any humidity out. You get a house that is cold and clammy, and a compressor that short-cycles its way to an early death. In a humid climate, right-sizing matters more than capacity.