I am by no means an expert on cl. hydrangea, but I have had mine for almost 3 years and since it's growing pretty well, I must be doing something right. (I'm in 6b) I don't believe it could be too little (or too much) sun...mine gets about the same amount of sun as yours and is not browning. I find it hard to believe it could be too much water...mine thrives on all the water I give it (I water it about 3-4 times a week, and my soil is just average, with some clay). I would guess too little water--but maybe you have a bug infestation? Not sure if this could cause that kind of problem, but mealybugs seem to like my plant--I also wonder about spider mites. I have had serious spider mite damage in the past which has killed off parts of some plants in my yard. Maybe if you posted a pic, we could see what the damage looks like and someone can tell you more.