What exactly is the soil mix? Looks fairly organic, so not necessarily boon-like if it is. Also, what are you fertilizing with? Does it include the needed micro-nutrients (stuff other than NPK)? I like organic ferts. like most, but many don't have all the micro-nutrients the tree needs. I've been using Foxfarms GrowBig weekly, in addition to an organic pellet on the soil surface. GrowBig organic based, but with some added punch so it covers the micro-nutrients.
Lastly, how hard is your water? Hard water can interrupt the plant's ability to use the fertilizer you give it. I had it so bad I moved to an RO water system for the bonsai. Makes all the difference for me.