@Dscholfield , I've had the same problem. My ski would always pop off the first wake primarily because of a flat ski.
However, I just got back from ski school last week and they noticed that in addition to a flat ski, I also was too far back on the ski. You are going to "ramp" the first wake more if you don't have enough ski in the water.
If you feel that you are on edge enough through the wakes, then your problem could be that you aren't stacked enough. Get some more weight on that front foot and see what happens.