Every Texas neighborhood has them: the house where the master bedroom or the "bonus room" above the garage is seemingly incapable of cooling down below 78°F, even when the thermostat in the hallway reads 72°F. You are not going crazy; you are fighting a localized thermal collapse.
The Diagnostic Trifecta
The master bedroom is frequently built furthest from the central AC air handler. The flexible duct pushing air to that room is often 40-feet long and winding through a 140°F attic. By the time the 55°F air travels that distance, it has baked up to 68°F. The "cooling velocity" is dead on arrival.
Master bedrooms frequently feature vaulted or tray ceilings. Builders hate insulating these angular cavities because they are extremely difficult to reach. The result is a massive, uninsulated radiant heat pad directly over the bed, pushing 120°F down into the room 24/7.
If there is no large return-air grill inside the master bedroom, the door acts as a dam when closed at night. The AC physically cannot push cold air into the room if it cannot simultaneously suck the hot, stale air out.