Help! Chinese Sweet Plum turning brown

[Tiberiusjones]

I bought this bonsai from Brussels and it arrived all green and very healthy. I had it sitting indoors by a window and I noticed the leaves at the top in the center were starting to turn very dry. I use the wooden coffee stirrer method to check for moisture and the soil was quite moist so I didn't water it except when it needed it. I thought maybe it was getting too much light so I moved away from the window in direct sun to an indirect lighting location. Same thing, leaves in the middle top turning brown. I moved it outside with my other bonsai under shade cloth and gave it a little water as it was starting to get drier. We have had plenty of rain this summer. Do you have any suggestions as why this happening and what to do to remedy it? I dont see any evidence of insects or fungus, I took the precaution of dosing it with some 3 in 1 when it arrived. I haven't had problems with that product with my other bonsai. I keep a little humification tray with gravel under neath the pot to help with watering but its not sitting in water ever. I was bad about overwatering in the past but the coffee stirrer trick solved that problem.