Depending on your height and weight, 18 may be too slow. My friend had a lot of problems clearing the wake until he finally let me speed up to 20 - 21 and now he clears it every time. It doesn't matter if the wake is big if you don't get enough air to clear it. Shorten the rope to 55' and once you can clear it at that length consistently then go back to 60'.
And the advice that Chase gave is completely true, make sure you're getting a good pop.