Oh that! You just have to make sure you're not touching a curved side of the concave pool piece. Usually, I put down everything else first and then add the concave pieces, the trick is the think of everything as squares, they looked curved but the footprint seems to register as a square, so there are a few placements in that. I know it sounds really confusing but if you get to thinking of everything as covering the square shape equivalent, and using the concave pieces last, it gets easier. Plus, mixing and matching square pools with curved pools works well too. In this, none of my concave pieces are touching each other, I have some regular square pool between them. That tends to help from keeping the "cannot place" message popping up. Does that makes any sense?