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