A great many red leaved Japanese maples will 'bronze out' or develop more green than red tones if in inadequate sunlight. And just as many will do the same when planted in full sun.........it is a bit of a conundrum to find just the right setting to encourage the leaf color you want!! Online forums are full of these reports of red leafed maples greening out - it is an extremely common occurrence. Even those cultivars that are reputed to hold their color well can experience this gradual leaf color change. Often it is just matter of time for full establishment before the tree returns to more normal colored leaf production but sometimes it is just an issue with the site itself and this will be a recurring issue each summer.

But there is nothing wrong with your tree. I'd just be inclined to give it a few years to establish properly and see if the color doesn't improve.