My ski partner struggles inconsistently. Things that help;
Ski size
Ski angle in water (don't get the ski too vertical, maintain some angle so it climbs out of the water)
Force against balls of front foot, don't use heel or back foot.
For driver;
Very different than wakeboarder. I use a very easy and progressive pull to get the skier moving and climbing out of the water. I've hardly moved the throttle off idle to get the skier moving