Could be root bound or could need some ironite.It also may not be getting as much water as you think,especially if it's root bound.Can you pull it up out of the pot a little so you can check the roots??
Kathi

Thanks for your response, Kathi. It's not root bound; we transplanted it in a plenty large size pot. The strange thing is that we have another identical plant, same kind of pot, same watering schedule, same light. It is doing fine and not losing any leaves. We did use sevin on both plants for bugs, but as i said, the other plant is not suffering at all. Could this somehow cause defoliation on one plant but not another?

Sevin should be safe on the plants. Try watering with a gallon of water with a teaspoon of dish soap in it. That will make the water stick to the soil. Once a plant gets dried out completely, sometimes the soil won't saturate itself well again.

Check the soil to see if you have fungus gnats crawling around in there. If so, the soap treatment should kill them.

If they are in pots in full sun, they should be watered every day, especially if they are in clay pots.