I have only been growing maples for about 1 year so I am no expert. I had the same problem with some of my seedlings. I was fertilizing them with too much bloodmeal. The bloodmeal is calcium nitrate which raised the ph of the soil in my pots. At higher ph's it can make minerals like iron unavailable to the plant. Iron may be unavailable at higher ph's and cause chlorosis. Vertrees says in his book on Japanese maples to give the plant proprietary iron solution as a temperary fix. He advises using an acid compost to lower the ph to a more acid mixture which should fix your chlorosis problem in the long run.

thanks!