Scratch the bark off with your thumb nail in the limb where there is no growth. If it is brown or yellow then it is dead and you will have to cut it off. If green then leave it, it might just be that the buds are taking longer to break - one of mine is still in bud whilst the others are in leaf. Die back does happen with Acers though it tends to happen more with younger ones. I lost half of a variegated one last year due to the heavy rain and the severe snow weather.