I struggle with this one too and have for years.
I case most of the time, and my board also slips out from under me. In my case, it's because I roll the flip off the wake early, instead of popping Up, and then throwing th trick. I know exactly what the problem is....just a brain fart! Even when I land them it feels all wrong.
You might consider whether or not you're hucking it earley too.