Japanese maples are beautifully ornamental, but can need some special care to do well. It has been very dry for the last month and some are suffering some scorching on their leaves, as their roots are shallow. Drip irrigation or supplemental water in times of drought is helpful.
(It hasn't rained since about mid-September in our area).
There are also diseases and insects that these trees are susceptible to.
We'd suggest having a tree health expert come to you and do an on-site evaluation of all the factors, and make suggestions for treatment. These experts, called Certified Arborists can be found by searching at www.treesaregood.org