There are two problems. First, you're using the wrong kind of maul. You need a larger maul with a triangular head, not the old-fashioned maul you use with wedges. The brand U grew up using was the Monster Maul, that appears to be no longer available. Bailey's has a similar product, but it's pricey. Anyway, this is the type of maul you need:
Bailey's - Steel Handle 14.5 lbs. "Mega" Mule Maul
You need that extra weight to get the enrgy you need into the wood, and the triangular head is much more efficient than a wedge and maul combo.
Second, if the maple is too hard to split, it means it hasn't cured ling enough. Let it dry another year or two and it should split fine.