What do you do if central air conditioning ducts are not blowing out strong enough to cool upstairs?

what needs to be done to put new unit with proper calculations for cooling load and static. Sometimes it cannot be achieved due to arrangement of the unit may be connected with other spaces as well.

Than in this case it is a specific study but can be handled

Check the following for possible causes of the problems.

1. Check the settings of the main A/C unit if it is chilled water system check if the unit is delivering the correct chilled water temperature.

2. Check all the valves connection to the air handling unit find out if all is properly open, the control valves must be properly adjusted or calibrated, open the strainer for possible dirt cloging the pipe supplyig the AHU.

3. Check the operating setting of the AHU the thermostat, the fan if it is belt driven see if the belt is loose, check also the power supply if the amp.and voltage delivered is correct.

4. Check acoustic lining in the duct sometimes is not properly fix and obstructing the duct opening.

5. Check if the duct sized installed is correct and according to approved plan design.

6. Check if the duct connection is correct and according to SMACNA standard.

7. Check for any leaks in the duct.

8. Check for any close supply dampers.

9. Maybe the duct system is not properly balance get an expert and try to balance again.

10. Get an expert maintenance, commissioning engineer and technician to solve the problem.

If you don't have any damage in your system visually, that means there is a fault in load calculation and may be in static pressure so you have to review all calculation and sizing first to know what's wrong with your system your question is a general question so i couldn't give an accurate answer kindly tell more details.

If the computation has made confirmed and correct for the duct size and static pressure. If the case check the AIR HANDLING UNIT. If it is giving also a correct air flow to supply in the entire confined space. If there is only a portion of the entire confined space is insufficient of air flow. Check all necessary check on the damper such us manual damper, vav damper, and fire damper.

Question is not clear, I want to know the problem is either we are not getting correct air flow at air outlet or comfort conditions are not achieved.

In case if required air flow is not achieved we must do air balancing again, may some broken split damper or due to duct construction air distribution is not correct, if the AHU is throwing required amount of air it means some problem in ESP, any way proceed in this direction may solve the issue.

In case air flow is okay but comfort conditions are not achieved than we have to check chilled water flow and inlet temperature.

firstly check the leakage of duct. 2nd clean up the duct inside. 3rd check the damper of the duct that is open properly. 

The same problem we suffer in one of the  3 bed room villa in bahrain.there are  more probable reason is 

due to as follows...

1. Blockage in the duct  near branch mouth by acoustic lining .Or if damper is available check the throttle position of the Damper.

2.not enough pressure buildup inside the duct due to leakage in the duct.

3.check the equipment blowing  CFM. and balance it per required.

in order to solve this problem the most efficient way is to put return in upstairs area as in your case the flow will automatically start rotating in upstairs