Because the ducts, not the equipment, usually decide room temperatures in these tracts. Long attic runs to the back bedrooms lose air at aging joints, pick up attic heat through thin or slipped insulation, and were often undersized at the original build. The test is an airflow read at each register with the system running. If the back registers move half the air of the front ones, the fix is duct sealing, resizing, or insulation work, and a bigger condensing unit will only make the front rooms colder while the back stays behind.
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