I think I started removing prism diopters too quickly.
I notice that when I'm at the right level of prism adjustment, the exercise with the fixation cards is a very different thing from what it is when I don't have enough. I get really close to fusion. And fusion/not fusion is a binary thing. You're either fusing or not. So there is a chasm of a difference there.
When there's not enough prism, I have to focus a lot on accommodation, which really defeats the point. I suspect that I might have wasted a few months of vision therapy because I jumped the gun with the prism reduction.
I think I'm going to go back to the four diopter prism and really practice fusion, and I'm not going to move forward until I can still get fusion comfortably with a lower diopter prism.