It's probably root-bound and needs a larger pot and the roots loosened. After you get that job done, soak it in an epsom salts and water solution (1 TBSP. ES to gallon of water). Its roots will settle in about two weeks from now, and you can then begin adding fertilizer to your water at 1/4 strength.

My Tiboucina is also losing it's leaves, which turn bright red before falling. I will try repotting as you are probably right, it is root-bound.Thanks- its worth a try.

to Kay Jones

I repotted my Tibouchina, it was very root bound, and took a long time to free all the roots soaking them continuously. It have continued to lose leaves , but this time they are turning yellow and falling, looking like an Autumn drop. Is it a deciduous shrub? I am tempted to prune it well to give it breathing space, but all the leaves would be cut off as they are only on the tips, would that matter? Help! Jill