Why does my new Japanese Maple tree turn red to green in fall?
The loss of red color changing to green of your Japanese maple (Acer Japonicum) tree leaves is probably due to the tree being in too shady a location. If it showed good color in the past in the same location then the problem is probably some environmental factor, e. g. too much nitrogen. A remote possibility is that a green-leaved sport has taken over the tree if a more mature tree. But full light to shadier condition of the sun is most likely the reason for color change.
Courtesy of the NYBG Plant Information Service