Certified GKH Gardening Expert
This will, almost always, have two causes. Lack of humidity, and lack of light. If you want this to fruit indoors, you will have to have supplemental light. These will require at least 200 watts per tree. I recommend, and use myself, a good LED panel. These will give them good light, and keep the cost to light it down.
It is also helpful to know that you will want to have, both, dolomitic lime, and iron sulfate handy. You will want to apply these once every few months. This will solve many problems before they happen.