Hey BB,
i Dont profess to be an expert in the slightest, But when you say that you water when the top soil becomes dry, I take it you would have checked to see if the mid to bottom areas are still moist, I am doing a stealth grow at the moment and One of my pots is like that , where the top soil looks dry but underneath it is still has plenty of moisture, this is also under CFL lights, I use a Soil mix designed for Bonsai, it has slow release nutes and also a wetting agent (Penetrade), so I am just wondering is your plant getting to much water, Just a thought.