thams
Omono
I have a pretty good hunch that the light you're using doesn't put out enough foot candles, lumens, or lux, depending on how you measure light production, to really give the tree what it needs. Also the color temperature of the light may not be in the correct spectrum. Then you have to consider that it doesn't spread light out over the entire tree relatively evenly, if its actually mounted close enough to the leaves not to waste the light output (which diminishes drastically the further away the light is).
That what I meant -- only you said it much better!