Check on the soil moisture - sometimes potting mix gets so dried out that the water you put on it isn't very well absorbed. If you've had it indoors in the a/c all summer, this may be what's happened. When you water it, does the water rush right through and come out the bottom of the pot right away? That's a sign.
You could put it on some newspaper (or take it outside), tip it on its side and slide it out of the pot to take a look at the root ball. If it's dried and solid like a brick you need to give it a really good, long soak in a big bucket (or a plastic kiddie pool works well, too) until that root ball is completely moist again. The tree will love it. A really thorough hosing down if you take it outside might accomplish the same thing, but be SURE that the pot weighs a lot more after you've soaked it than it did before.
