My UPS doesn't provide proper backup power when the line power goes out

My UPS doesn't provide proper backup power when the line power goes out

Explanation: A UPS is being used on the PC, but when the power goes out the UPS doesn't keep the PC running.

Diagnosis: There is either something wrong internally with the device, or it has been connected or configured incorrectly.

Recommendation:

If the unit is making any noises or has LEDs flashing in strange configurations, consult the manufacturer's technical support.
The unit may be overloaded. If a 400 kVA unit will support your system for 5 minutes on battery backup, this does not mean that a 200 kVA unit will necessarily support it for 2.5 minutes. Once a unit gets too much load on it, it may not work at all when the power goes out. You will need a more powerful unit in this case.
Check the unit physically to make sure that everything is connected properly, there are no tripped circuit breakers, etc.
Try plugging the PC into a different socket in the device and see if that solves the problem. If it does, the device has a failure with that particular socket and it needs to be repaired or replaced.
The batteries could be damaged, defective, or discharged. Follow the manufacturer's instructions for ensuring that the batteries are fully charged and see if the problem works properly.
Consult with the manufacturer of the device for technical support.