My
problem with popups has more to do with my strength than it does with my technique. If I'm tired, then my popups start to slow down and my back foot can drag slowly into place. In other words, my technique changes (deteriorates usually) based on my fatigue level. Anyway, the single most important aspect for me when I'm popping up, especially on steep wave faces, is getting a feel for what Martin Dunn calls "the ledge". In the following video he describes it. The instant he used the word "ledge" in the video I knew exactly what he was talking about. I had been experiencing the ledge, but never had a way to think about it. Sometimes when I am able to describe things, they become more intuitive to me when I encounter them in whatever I'm doing. Maybe the video will help you too.
Anyway, when I first take off on a wave and my board locks into the wave's momentum, I can feel that ledge just as I'm about to pop up. My popup will usually succeed if my board is horizontal (still flat) in those first moments I'm in the ledge zone, but if I'm slow at the top of the ledge and my board starts to angle too far down the wave and pick up downward momentum, then my popup tends to fail. You may be beyond that mistake, but I find that's often where my popups usually go wrong. The success of my popups depend on my fatigue level and the verticality of the wave face. Here's the video.