There are multiple reasons and when your air con would not feel cold. Lets look into a few scenario.

• When you recently purchase your NEW air-conditioner but you are feeling not cold enough 
  1. It could be undersize where your capacity is insufficient to meet the cooling load of your room
  2. It could be insufficient refrigerant that cause insufficient cooling to your room
  3. If you recently change from a non-inverter aircon to an inverter aircon, you might feel the cooling difference as non-inverter aircon runs at 100%

• When only 1 of your aircon unit has been doing a good job but suddenly dies on you (The other indoor units are working properly)
  1. The thermostat sensor is faulty and giving a wrong signal back to your aircon system
  2. The aircon unit has not been service for a long period of time, it could possibly be choke with dirt and dust.

• When all your air-con is not feeling cold
  1. The compressor has break down – check if the compressor is running by listening to it at the outdoor unit
  2. There is a leak in your piping and cause insufficient refrigerant gas to cool down the room
  3. Outdoor unit fan motor faulty
  4. Outdoor unit PCB faulty