It's probably just the way it is, especially if it was grown from seed and not a grafted cultivar. You're not doing anything wrong. Different japanese maples do differnt things like that. Some are red year round, some do what yours is doing, etc. Different amounts of light can play a role too. For example, I have a Burgundy Lace japanese maple in a pot that I kept in complete shade last year, and it was green all year until fall. This year I moved it to a little more sunnier spot and it has been completely red this season.