Why can't I reach my most complex-needs students?

You have learned to adapt. That is real progress, and you should not dismiss it. But adapting means you still start from the typical lesson and stretch it. For your most complex students, stretching is not enough. The gap between their needs and your plan is too wide to close by bending the same shape. Here is the harder truth: you are calling this a skill problem because that flatters you. A skill problem means you just need more practice, more patience, more tricks. But you already have skill. What you lack is a different starting point. You are designing for the middle of the room and patching the edges. The edges need their own design. This will not resolve itself with more effort at what you already do. It requires you to build backward — from the student who struggles most, outward to the rest. That is uncomfortable, because it means your first draft of every lesson is no longer for the majority. Choose to accept that discomfort, or keep circling the same wall.