Your aircon capacity probably too high for the room. Room cools down faster than it can dehumidify. Like others said, only thing you can do is to run at minimum fan speed, but this isn't that good for the ac long run coz it might risk condensation on the trunking due to the very cold pipes and slow heat transfer at slow fan speeds.
Duct wise, the only time that it can go upwards if they have a drain pump, but generally not supposed to go against gravity.
Interesting, always thought the cooling and dehumidification goes hand-in-hand. So it appears that it can actually cool too fast (blower moving too much air too quickly?) and dehumidification rate cannot keep up with it.

