Solanum rantonnetii

[Colemanjan]

I have two large Solanum Rantonnetii (Blue Potato Bush) which have been pruned to a nice small tree with a circular canopy. One is younger plant with a 3 inch trunk, the other is oldre with a 5 inch trunk. The polder tree is not doing well. Its losing leaves and is bear in all areas except the very top. There is no visible problems by looking at the leaves (either dropped leaves or the ones still in tact), there is no visible sign of fungus in the soil. Both plants are watered the same way, pruned the same way and get monthly dose of general time release fertilizer. I have tried giving the sick tree some B-1, which helped some. I live in Los Angeles, and the plants are both exposed to good sun light.
Like to save this old bush, can some one suggestions??