Stairs still warping.

Even after letting these planks age in the sun the whole summer, then going back and remilling them, now that I've got them in place in the wall, they are still warping more. how frustrating. I think it actually may have to do with the water content in the plaster... somehow they are soaking up the water and using that move away from flat.

I thought this picture would show it well, but the lower step is pulling up, even cracking the plaster below it, and the upper step is sloping down, especially on the left side. They can't even warp the same way?!?