Look on the bottoms of the leaves for small, soft round shaped bugs. Those are aphids. If you have them, find a systemic for aphids. Anything you spray on a Japanese Maple will damage it - they are just too sensitive.
Don't fertilize with anything high in nitrogen as that will also cause damage. Low dosages of balanced fertilizer once or twice a year is really all it needs. This time of year, JMs do start to look raggedy so if you don't find insects... Have you had any hot dry winds? That usually finishes mine off for the year.
