This variety of Acer has a strong inclination to revert, meaning it produces branches with non variegated leaves. These should be cut out as soon as they are seen, at the base of the particular all green branch where it arises from the trunk or bough. If you're saying all the leaves are plain, then its likely you didn't notice this happening and that the reverted growth has taken over, because its much more vigorous than the variegated growth. Inspect the tree thoroughly to see if you can find any variegated growth at all - if there's enough, cut out the non variegated.