Is it in heavy shade or cramped by other shrubs? Acers like to have space & the red varieties seem to colour better if they have a little sunlight at some time during the day.
It gets sun for a large portion of the day - it is on the South side of the house. I read something about them grafting red maples to green japanese maple tree stems for better growth. Could the green leaves be coming from the grafted tree stem?
The green bit will be more vigorous than the red, so if it does look like a graft (you will be able to see the join) try and prune the green bits off, if you can do it without making it look odd.