When one room never keeps up, the instinct is to blame the unit outside. More often the problem is between the air handler and that room's register, in an attic that spends the summer well above 130 degrees.
Flex duct has an inner liner, a layer of insulation, and an outer jacket. Fifteen South Texas summers degrade all three. The liner tears at the collars, insulation compresses where the run has been stepped on or left sagging over a joist, and connections that were only taped instead of mastic-sealed let go entirely. Air that leaks into the attic is air you paid to cool and will never feel.
Sagging is its own problem even when nothing has failed. Every dip and kink adds resistance, and a run that snakes instead of running straight can lose a large fraction of its delivery. Undersized returns do the same thing from the other direction: the blower can't pull enough air across the coil, static pressure climbs, and the whole system moves less air while working harder.
There are signs you can check without going into the attic. Registers that barely move a tissue, rooms at the far end of the house that run several degrees warmer, a system that runs constantly but never satisfies, and a noticeable dust problem are all consistent with duct losses.
The fix is not glamorous and it is not usually a full replacement. Sometimes it's resealing connections and adding return capacity. Sometimes it's replacing two bad runs. What matters is that someone gets up there, photographs what's actually happening, and prices the duct work honestly next to the equipment instead of quietly ignoring it.