From your question, I understand that the problem is started for few weeks only. There were no problem before. So that means your existing design for the selection of capacitor bank is correct. Now, how did you find out that power factor is going less. From the Pf meter?...The solution for your problem is not to add capacitors. But you have to find out the reason for this and rectify it. Investigate the type of load and what are those load when you noticed low power factor. Inspect individually these equipments for example motors, UPS, flouresent lamps etc. If really the initial selection of your capacitor bank is correct, you can find the reason of problem from these equipments. Rectify it.....otherwise ill you add capacitors again, if tomorrow the power factor gone less again??, ( or you just posted for getting information of capacitance selection??